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OUTLINES   OF   THE 

COMPARATIVE    PHYSIOLOGY   AND 

MORPHOLOGY   OF   ANIMALS 


By  Prof.  JOSEPH   LE  CONTE. 


THE   COMPARATIVE   PHYSIOLOGY 

AND  MORPHOLOGY   OF   ANIMALS. 

Illustrated.     i2mo.    Cloth. 

The  work  of  Darwin  on  the  derivation  of  species  and 
the  descent  of  man  awakened  a  new  interest  in  the  lower 
animal i,  and  furnished  additional  evidence  of  their  close 
kinship  with  ourselves.  A  fresli  field  of  study  was  thus 
opened  up,  embracing  the  likenesses  and  differences  of  ac- 
tion as  well  as  structure  found  throughout  the  animal  king- 
dom. In  this  work  Professor  Le  Conte  gives  us,  in  his 
well-known  clear  and  simp'e  style  and  with  the  aid  of 
numtrjus  illustrations,  an  interesting  outline  of  these  simi- 
larit.es  and  variations  of  function  as  displayed  among  the 
various  classes  of  animals  from  the  lowest  to  the  highest, 
man  included. 

RELIGION    AND   SCIENCE. 

A  Series  of  Sunday  Lectures  on  the  Rehiion  of  Natural  and  Re- 
vealed Religion,  or  tne  Truths  revealtd  in  Nature  and  Scrip- 
ture.    i2mo.     Clolh,  $1.50. 

ELEMENTS  OF   GEOLOGY. 

A  Text-Rook  for  Col'ege?  and  for  the  General  Reader.  With  new 
Plates,  new  Illustrations,  new  Matter,  fully  revised  to  date.  8vo. 
Cloth,  $4  00. 

SIGHT. 

An  Exposition  of  the  Principles  of  Morocular  and  Binocular  Visior. 
With  Illustrations.  Second  edition.  Ko.  31,  International 
Scientific  Series.     i2mo.     Cloth,  $1.50. 

EVOLUTION   AND   ITS   RELATION  TO 
RELIGIOUS  THOUGHT. 

Revised  edition.     i2ino.     Cloth,  $1.50. 


D.  APPLETON  AND  COMPANY,  NEW  YORK, 


OUTLINES   OF 
THE  COMPARATIVE    PHYSIOLOGY 
AND   MORPHOLOGY   OF  ANIMALS 


BY 


JOSEPH    LE   CONTE 


AUTHOR   OF 

RELIGION    AND    SCIENCE  ;    ELEMENTS    OF    GEOLOGY  ;    SIGHT, 

AN    EXPOSITION    OF    THE    PRINCIPLES   OF    MONOCULAR 

AND    BINOCULAR    VISION  ;    EVOLUTION    AND    ITS 

RELATION    TO    RELIGIOUS    TH?)UGHT,    ETC. 


NEW   YORK 

D.    APPLETON   AND  COMPANY 

1900 


Copyright,  1899, 
By  D.   APPLETON  AND  COMPANY. 


fi?33 


PREFACE. 


So  many  books  have  recently  come  out,  and  are  still 
coming  out,  on  zoology  and  biology  that  it  seems  neces- 
sary that  1  should  say  something  of  the  reasons  for 
this  one. 

Nearly  all  the  books  now  coming  out  are  devoted, 
and  rightly  so,  to  practical  laboratory  methods,  and 
especially  to  the  study  of  selected  types.  This  method, 
first  introduced  by  Rolleston  and  rendered  popular  by 
Huxley,  was  a  reaction  against  the  barrenness  of  the  old 
text-book  and  lecture  method.  It  was  certainly  timely 
and  necessary  ;  but  there  is  danger  that,  like  all  reac- 
tions, it  may  have,  and  indeed  has  already,  gone  too  far. 
Undoubtedly  the  teaching  by  types  is  indispensable  in 
the  early  part  of  the  course,  in  order  to  introduce  the 
student  into  the  true  spirit  and  methods  of  science  ;  but 
to  continue  it  and  "make  it  the  main  form  of  teaching 
is  a  serious  mistake."  *  There  is  serious  danger  that  in 
the  attempt  to  explore  thoroughly  a  few  small  spots 
here  and  there  in  the  field  we  lose  sight  of  the  general 
connection  of  all  parts  to  one  another  and  to  the  whole 
— that  in  the  microscopic  clearness  but  narrowness  of 
our  knowledge  we  lose  that  general  view  of  the  whole 
which  alone  gives  significance  to  any  knowledge. 

Such  a  general  view  of  the  physiology  and  morphol- 
ogy of  the  animal  kingdom  is,  it  seems  to  me,  a  great 

*  Lankester,  Nature,  Iviii,  25,  1S98. 


vi       PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

want  in  our  teaching  of  zoology  to-day — a  want  which 
is  only  now  beginning  to  be  recognized.  It  is  just  such 
a  general  view  which  I  have  attempted  to  give  in  this 
volume.  The  book  is  not  intended  to  take  the  place  of 
books  already  in  the  field,  but  to  supplement  them.  It 
is  intended  to  precede  and  accompany  the  special  labo- 
ratory courses  of  our  high  schools,  colleges,  and  univer- 
sities. It  must  itself  be  preceded  by  the  type  method  in 
the  schools. 

Some  will  think  I  have  too  much  slighted  the  inver- 
tebrates. I  can  only  say  that  this  was  unavoidable  if  I 
kept  within  the  limits  of  a  moderate-sized  book.  I  have 
given  only  what  every  intelligent  person  would  like  to 
know. 

Again,  some  will  perhaps  think  that  I  dwell  too 
much  relatively  on  certain  functions — e.g.,  the  sense  of 
sight  and  glycogeny.  I  can  only  answer  that  a  perfectly 
balanced  treatise  on  any  wide  and  complex  subject  is 
well-nigh  impossible,  and  I  am  not  sure  that  it  would  be 
best  even  if  it  were  possible.  A  certain  insistence  on 
points  best  known  to  and  most  thoroughly  investigated 
by  the  teacher — a  certain  hobby  riding,  if  not  carried 
too  far — is  necessary  to  give  life  and  interest  to  any 
subject. 

Some  may  object  to  the  order  of  treatment — descen- 
sive  instead  of  ascensive.  This,  I  believe,  finds  justifica- 
tion in  the  fact  that  physiology,  not  morphology,  is  the 
prominent  point  of  view.  This  I  explain  fully  in  the 
book  (page  27). 

The  work  is  the  final  embodiment  of  a  course  of  lec- 
tures continued  and  compacted  for  many  years,  and 
given  in  connection  with  and  preparatory  to  the  labora- 
tory courses  in  zoology  in  the  University  of  California. 

Joseph   Le  Conte. 
Berkeley,  Cai..,  October,  iSgg. 


CONTENTS, 


CHAPTER    I. 

INTRODUCTORY. — SOME   GENERAL    PRINCIPLES. 

SECTION   I. 

Relation  of  the  Three  Kingdoms  of  Nature. 

Living  vs.  IVonliving,  i. — (i)  Organization,  (2)  cellular  structure, 
(3)  growth,  2.     (4)  Life  history,  (5)  reproduction,  (6)  metabolism,  3. 

Animals  vs.  Plants,  i,. — (i)  Sensation  and  volition,  (2)  nature  of 
food,  5.     (3)  The  posession  of  a  stomach,  6.     (4)  Waste  and  supply,  7. 

SECTION    II. 

Definition  of  zoology,  8.  Divisions  of  zoology  :  (i)  comparative 
anatomy,  (2)  comparative  physiology,  (3)  comparative  embryologv,  (4) 
taxonomy,  9  ;  (5)  descriptive  zoology, '6)  paleozoologv,  (7)  geographical 
zoology,  10  ;  this  course  consists  mainly  of  second  and  first,  11. 

SECTION    III. 

General  Cellular  Structure  of  Animals. 

Definition  of  cell,  animal  vs.  vegetal  cell,  11.  Size,  softness,  trans- 
parency, differentiation,  12. 

Tissues. 

Definition,  12.  Kinds  :  (i)  Connective,  13  ;  (2)  cartilage,  (3)  bony, 
15  ;  (4)  muscular,  17  ;  (5)  nervous,  19  ;  (6)  epithelial,  20.  Law  of 
differentiation  of  cells,  22. 

SECTION   IV. 

Organs  and  Functions  of  Animal  Body. 

Classification  of  function,  23.  Animal  functions  defined,  organic 
functions  defined,  subdivisions,  order  of  treatment,  24. 

vii 


Vlll    PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 

PART    I. 

ORGANS  AND    FUNCTIONS  OF  ANIMAL   LIFE. 

Four  systems  of  organs  concerned,  26.  How  they  co-operate,  or- 
der of  treatment  justified,  27. 

CHAPTER    L 

NERVOUS   SYSTEM. 

Two  subsystems  (cerebro-spinal  and  ganglionic),  cerebro-spinal  sys- 
tem of  vertebrates,  general  plan  of  and  subdivisions.  29. 

SECTION    I. 

Brain  of  Man. 

Skull,  membranes,  30.  Main  parts  of  brain  :  (i)  cerebrum,  31  ; 
(2)  cerebellum,  (3)  medulla  and  pons,  (4)  optic  lobes,  32  ;  (5)  thalamus 
and  corpus,  33.  Convolutions  of  cerebrum,  of  cerebellum,  34.  Interior 
structure,  microscopic  structure,  35.  Embryonic  development  of  brain, 
37.  Fore  brain,  mid  brain,  and  hind  brain,  39.  Distinctive  functions 
of  cerebruiTi  39  ;  of  cerebellum,  of  medulla,  40  ;  optic  lobes,  thalamus 
and  corpus,  41.     Localization  of  cerebral  functions,  43.     Dextrality,  45. 

SECTION    II. 

Spinal  Cord  0/  Man. 

Envelopes,  description  of  the  cord,  spinal  nerves,  46.  Section  of 
the  cord,  47.     General  function  as  conductor  and  as  center,  48. 

SECTION    III. 
Nerves. 

Cranial  nerves,  49.  (i)  Olfactory,  (2)  optic,  50  ;  (3,  4,  and  6) 
oculi  motores,  52  ;  (5)  trigeminal,  (7)  facial,  (8)  auditory,  (9)  gustatory, 
(10)  vagus.  53  ;  (11)  spinal  recurrent,  (12)  hypoglossal,  general  obsei^va- 
tions  on  cranial  nerves,  54. 

Spinal  nerves.  Origin  and  distribution,  54.  Structure  of  nerves, 
function,  56.  Mode  of  action  illustrated,  two  subsystems,  57.  Course 
and  termination  of  fibers,  58.  General  mode  of  action  of  whole,  59. 
Course  in  reflex  action,  60.  Illustrated  by  telegraphy,  61.  Applica- 
tion to  several  cases,  62.  Law  of  peripheral  reference,  64.  Nerve 
force  vs.  electricity,  64.     Function  of  spinal  or  reflex  system,  66. 

SECTION   IV. 

Ganglionic  System. 

Definition  and  description,  67.  Principal  plexuses,  function  of 
ganglionic  system,  69. 


CONTENTS.  ix 

SECTION   V. 

Comparative  Physiology  and  Morphology  of  Nervous  System. 

Introductory. — Outline  of  the  classification  of  animals,  70. 
Comparative  Morphology  of  the  Vertebrate  Nervous  System. — Gen- 
eral plan  of  structure,  72. 

Brain  0/  Vertebrates. 

(i)  Variation  in  size,  absolute,  72  ;  and  relative,  73.  (2)  Relative 
amount  of  gray  matter,  74.  (3)  Relative  size  of  cerebrum,  76.  Owen's 
classification  of  mammals,  77.  Pineal  gland,  78.  Embryonic  and 
taxonomic  series  compared,  79.  (4)  Relative  size  of  frontal  lobe, 
82.      Cephalization,  82. 

SECTION   VI. 

Nervous  System  0/  Invertebrates. 

1.  Articulata. — General  plan  of  structure  compared  with  vertebrates. 
84.  General  plan  of  nervous  system,  85.  Functions  of  the  several 
ganglia,  86.     Modifications  in  going  down  and  up  the  scale,  87. 

2.  Mollusca,  Sg. — General  plan  of  nervous  system,  90. 

3.  Radiata. — General  plan  of,  92. 

4.  Protozoa  have  no  nervous  system,  93. 

CHAPTER    II. 

SENSE   ORGANS. 

SECTION   I. 

Introductory. 

Relation  of  special  sense  to  general  sensibility,  94.  Illustration  of 
the  law  of  differentiation,  95.  Gradations  between  the  senses:  (i) 
In  perception  of  vibrations,  96  ;  (2)  in  kind  of  contact,  (3)  in  objec- 
tiveness,  97.     Higher  and  lower  senses,  98. 

Sense  0/  Sight  and  its  Organ,  the  Eye. 
Primary  divisions  of  the  subject,  98. 

SECTION   II. 
Eye  0/  Man  —  General  Structure. 

Shape,  setting,  99.  Muscles,  100.  Coats  of  the  ball,  loi.  Lin- 
ings, contents  or  lenses,  102. 

Forynation  of  the  Image,  103. — Necessity  of  lenses,  104.  Applica- 
tion to  the  eye,  105.     Proof  of  retinal  images,  ic6. 

Comparison  of  the  Eye  and  the  Camera. —  Defects  of  lenses  and 
their  correction  :  (i )  Chromatism,  107  ;  (2)  aberration,  108.  Accom- 
modation, Helmholtz's  theory  of,  109.  Adjustment  for  light,  iii. 
Structure  of  iris,  112. 

Defects  of  the  Eye  as  an  Optical  Instrument. — Normal  sight — em- 
metropy,  myopy,  113.     Hyperopy,  presbyopy,  114.    Astigmatism,  115. 


X        PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

SECTION  in. 

The  Retina  and  its  Functions. 

Structure  of  the  retina,  117.  Its  layers,  118.  Bacillary  layer,  dis- 
tribution of  the  rods  and  cones,  iig.  Distinctive  function  of  the  lay- 
ers, 120  ;  of  the  rods  and  cones,  121.     Visual  purple,  121. 

SECTION   IV. 
Perception  of  Space  and  0/  Objects  in  Space. 

First  Law  of  Vision.  Law  of  Spatial  Reference  of  Retinal  Impres- 
sions, 122. — Comparison  with  other  senses,  123.  Illustrations  of  the 
law  :  (i)  Irritation  of  retina,  (2)  phosphenes,  (3)  muscse  volitantes,  124. 
(4)  Purkinje's  figures,  (5)  ocular  spectra,  125  ;  generalization,  126. 

Second  Law  of  Vision;  Law  of  Direction,  126. — Comparison  with 
other  senses,  128.  Explanation  of  some  visual  phenomena  :  (i)  erect 
vision,  explanation,  129.  (2)  Fovea  and  its  spatial  representative,  130  ; 
minimum  visibile,  131  ;  compare  with  touch,  132.  {3)  Blind  spot, 
132  ;  experimental  proof,  133  ;  spatial  representative  of,  134. 

Color  Perception  and  Color-blindness,  135. — Intensity  vs.  color, 
primary  colors,  136.  Theory  of  color  perception,  general  theory,  137. 
Special  theories,  138.  Color-blindness,  139.  Cause  of  coloi-blind- 
ness,  what  the  color-blind  see,  140.     Tests  of  color-blindness,  141. 

SECTION   V. 
Binocular  Vision. 

Definition,  142.  Single  and  double  vision,  142.  Experiments  illus- 
trating double  vision,  single  vision,  143.     Corresponding  points,  144. 

Third  Law  of  Vision;  Law  of  Corresponding  Points. — Conditions 
of  single  vision  illustrated,  144.  Iloropteric  circle  and  horopter  defined, 
146.  Relation  of  chiasm  to  corresponding  points,  147.  The  two  ad- 
justments of  the  eyes,  two  kinds  of  corresponding  points,  148.  Two 
fundamental  laws  of  vision,  149. 

Binocular  perspective,  experiments  illustrating,  149.  Limitation  of 
clear  vision,  151.  Different  forms  of  perspective  :  (i)  Aerial,  151  ;  (2) 
mathematical,  (3)  binocular,  (4)  focal,  152. 

Judgments  of  Size  and  Distance. — Distance,  size,  153.  Form,  gra- 
dations of  judgments,  155. 

SECTION    VI. 

Comparative  Physiology  and  Morphology  of  the  Eye. 

Vertebrates. 

Mammals:  Iris,  pupil,  tapetum,  156;  fovea,  157.  Birds:  iris, 
sclerotic  bones,  nictitating  membrane,  157  ;  fovea,  158.  Reptiles,  158. 
Fishes,  binocular  vision  in  vertebrates,  159.  Chiasm  in  vertebrates, 
position  of  the  eyes,  160.     Fovea,  161. 


CONTENTS.  XI 


Invertebrates. 

JSIollusca  :  Cephalopods,  gastropods,  163;  acephala,  164.  Arthro- 
pods :  Simple  eye,  164;  compound  eyes  of  insects  and  crustaceans, 
165  ;  origin  of  compound  eye,  167. 

Evolution  0/  the  Eye. 

(i)  Invertebrate  eye,  167.  Steps  of  evolution  illustrated,  168.  (2) 
Vertebrate  eye,  170.  Several  steps  of  evolution  illustrated,  171. 
Transition  from  the  invertebrate  to  the  vertebrate  eye,  172.  Further 
evolution  of  the  vertebrate  eye,  173. 

SECTION   VII. 

Sense  0/  Hearing  and  its  Organ,  the  Ear. 

Structure  of  human  ear,  exterior  ear,  mid-ear,  174.  Ossicles,  in- 
terior ear,  or  labyrinth,  176.  Bony  labyrinth,  membranous  labyrinth, 
178.  Membranous  cochlea,  179.  Mode  of  action  of  the  whole,  dis- 
tinctive functions  of  the  parts,  181. 

Comparative  Morphology  and  Physiology  0/  the  Ear. 

Mammals,  birds,  reptiles,  183.  Fishes,  invertebrates,  insects,  184. 
Crustacea,  mollusca,  185.  Mosquito,  lS6.  Diagram  illustrating  suc- 
cessive simplification,  1S7. 

SECTION   VIII. 

Lower  Senses. 

Sense  of  Smell. — Nostril,  iSS  Smelling,  igo.  Comparative  phys- 
iology of  smell,  keenness  of  smell,  how  judged  of,  191.     Invertebrates, 

193- 

Settse  of  Taste. — Analysis  of  this  sense,  mixed  with  feeling,  with 
smell,  194.  Examples  of  pure  tastes,  195.  Papilloe  of  the  tongue, 
196.     Comparative  physiology  of  taste,  197. 

Sense  of  Touch  — Analysis  of  this  sense,  mixture  of  many  sensa- 
tions, 198.  General  sensibility  vs.  special  sen^e  of  touch,  general  or- 
gan of  this  sense,  199.  Special  organ,  200.  Minimum  tactile,  double 
tactile  images,  201.  Comparative  physiology  of  touch  in  vertebrates, 
202  ;  in  invertebrates,  203. 

SECTION    IX. 

The  Voice  and  its  Organ,  the  Larynx. 

(i)  The  Simple  Voice :  Larynx,  its  position  and  relation,  204. 
Structure,  206.  Glottis  and  vocal  cords,  207.  Their  action  in  vocali- 
zation, 208.  Muscles  of  the  larynx  and  how  they  act,  209.  (2)  Song  : 
Larynx  as  a  musical  instrument,  210.  (3)  Speech,  211  :  Vowel 
sounds,  212.     Consonants,  213. 


Xll      PHYSIOLOGY   AND   MORPHOLOGY    OF   ANIMALS. 


Comparative  Physiology  of  Voice. 

Mammals,  213.    Birds,  syrinx,  214.    Structure  and  mode  of  action, 
215.     Reptiles,  frogs,  insects,  216.     Grasshoppers,  cicada,  217. 


CHAPTER    III. 

MUSCULAR   AND    SKELETAL   SYSTEMS. 

SECTION    I. 

Muscular  System. 

Muscles,  kinds  of,  220.  Voluntary  muscle,  221.  Structure  of  in- 
voluntary muscle  and  its  mode  of  action,  223. 

SECTION   II. 

Skeletal  System. 

Defined,  223.  Number  of  bones  in  man,  joints,  movable  joints, 
224.  Examples  of  adaptation,  225.  (i)  Comparative  morphology  of 
vertebral  cohimn,  226.  (2)  Structure  of  shoulder  joint  and  fore  limb  m 
vertebrates,  motion  and  locomotion,  li?nb-»iotioii,  227.  Power  of  mus- 
cular contractions  shown  by  examples,  biceps,  228.  Deltoid,  gastroc- 
nemius and  soleus,  229.  Locomotion,  230.  Co-ordination  of  muscular 
action,  231. 

SECTION   III. 
Comparative  Morphology  and  Physiology  0/  Muscle  and  Skeleton. 

Vertebrates,  invertebrates.  Arthropods,  232.  Relation  of  muscle 
and  skeleton,  hinge  motion,  233.  Universal  motion,  234.  Worms, 
mode  of  locomotion,  235. 

Molhtsca  :  Acephala,  236  ;  gastropods,  cephalopods,  237.  Caelen- 
terata  :  Aledusce,  238.     Protozoa  :  Infusoria,  rhizopods,  239. 

CHAPTER    IV. 

GENERAL   LAWS   OF   ANIMAL   STRUCTURE,   OR   GENERAL   LAWS   OF 
MORPHOLOGY. 

SECTION   I. 

Introductory. 

Analogy  vs.  Homology,  examples  of  each  from  animals,  242. 
From  plants,  243.  Two  fundamental  ideas  in  homology,  246.  Ho- 
mology traceable  only  within  the  limits  of  primary  divisions  of  the 
animal  kingdom,  247. 


CONTENTS.  Xlll 

SECTION  II.  / 

Homology  of  Vertebrates. 

I.  General  Plan  of  Structure,  or  General  Homology. — General 
characteristics  of  vertebrates  :  (i)  Relation  of  muscle  to  skeleton,  (2) 
possession  of  backbone,  (3)  possession  of  two  tioink  cavities,  (4)  struc- 
ture of  head,  (5)  only  two  pairs  of  limbs,  strong  suggestion  of  common 
origin,  249. 

II.  Special  Homology. — Definition,  the  proof  of  common  origin,  best 
shown  in  limbs,  [a)  Fore  limbs  of  all  classes  compared,  250.  Hind 
limbs  of  different  orders  of  mammals  compared,  255.  Plantigrade, 
digitigrade,  unguligrade,  256.  Manus  and  pes,  classification  of  ungu- 
lates by  foot  structure,  257.  Rudimentary  and  useless  organs,  258. 
Homology  in  other  systems,  260. 

III.  Serial  Homology. — Definition  of,  serial  homology  of  the  verte- 
brate skeleton,  261.  A  vertebral  segment,  262.  Ov.en's  archetype, 
modifications  in  the  series,  263.  Origin  of  limbs,  264.  Serial  homol- 
ogy of  other  systems,  265. 

SECTION    III. 
Homology  among  Invertebrates. 

1.  Artictdata. — General  plan  of,  266.  Serial  homology,  267.  So- 
mite defined,  repetition  and  modification  of  somites,  268.  Illustrated 
from  crawfish,  269.  Crab,  modifications  in  going  down  the  scale,  271. 
In  going  up  the  scale,  272.  Origin  of  insects'  wings,  273.  Law  of 
differentiation,  homology  of  nervous  system,  276. 

2.  Molhisca. — General  plan,  explanation  of,  277. 

3.  Radiata. — General  plan  is  radiated,  278.  Comparison  with 
other  types,  279. 

4.  Protozoa,  279. 
General  conclusions,  280. 


PART    II. 

ORGANS  AND   FUNCTIONS   OF  ORGANIC  LIFE. 

CHAPTER    I. 

NUTRITIVE    FUNCTIONS — METABOLISM. 

Definition  of  metabolism,  283.  Waste  and  its  relation  to  life,  to 
work,  to  heat,  284.  Necessity  of  food,  285.  Necessity  of  waste- 
removal,  anabolism  and  katabolism,  286.  Three  divisions  of  the  sub- 
ject of  this  part,  287. 


XIV    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


CHAPTER    II. 

NUTRITION   PROPER — ANABOLISM — FOOD    PREPARATION — DIGESTIVE 
SYSTEM. 

SECTION    I. 

Food  and  its  Uses. 

Definition  of  food,  kinds,  288.  Uses,  289.  Distinctive  uses  of 
kinds,  waste  tissue  used,  290. 

Food  Preparation. — The  different  steps  of  preparation,  291. 

SECTION    II. 
Mouth  Digestion  in    Vertebrates. 

Salivary  glands,  292  ;  structure  of,  293.  Composition  and  use  of 
saliva,  294.     Ferments  defined,  295. 

Comparative  Physiology  of  Mouth  Digestion  in  Vertebrates. — Teeth 
in  vertebrates,  295.  Mammalian  teeth,  origin  and  development  of, 
296.  Composition  of  teeth,  kinds  of  teeth,  297.  Variation  in  teeth : 
in  size,  in  relative  number  of  the  kinds,  298.  Dental  formula,  299. 
Structure  of  molars,  300.  Of  herbivorous  molars,  origin  of  this  struc- 
ture, 302.  Mouth  armature  of  whales,  304.  Homology  of  baleen 
plates,  birds,  305.  Teeth  of  extinct  birds,  reptiles,  fangs  of  serpents, 
structure  of,  30&.  Origin  of  mammalian  teeth,  _/^j.//£j',  kinds  of  teeth, 
308. 

SECTION   III. 

Stomach  digestion — Chyvtiftcation — Peptonization. 

Saccharization  of  food,  310.  Stomach  described,  311.  Coats,  me- 
chanical work  or  chymification,  312.  Chemical  work  or  peptonization, 
313.  Composition  and  uses  of  gastric  juice,  effect  of,  on  milk,  ab- 
sorption from  stomach,  314. 

Comparative  Physiology  of  Stomach, 

Ruminants,  315.  Evolution  of  ruminant  stomach,  granivorous 
birds,  digestive  apparatus  of,  317.     Evolution  of  this  apparatus,  318. 

SECTION   IV. 
In testinal  Digestion — Chylification  —  Em  ulsification . 

Form  and  structure  of  intestines,  319.  Relation  to  abdominal  walls, 
320.  Mesentery,  peritonceum,  321.  Coats  of  the  intestines,  mechan- 
ical work,  322.  Chemical  work,  action  of  bile,  323.  Of  pancreatic 
juice,  emulsion  defined,  324.  Absorption,  325.  Two  modes  of,  gen- 
eral course  of  each  to  the  circulation,  326.  Sangiiificatioii,  '}2-j.  Effect 
of  the  liver  and  of  the  mesenteric  glands,  portal  vein  described,  328. 

Modification  of  process  of  intestinal  digestion  in  vertebrates,  329. 
Ccccum  in  a  rat,  330. 


CONTENTS.  XV 

SECTION   V. 

Digestive  System  in  Invertebrates. 

Arthropods. 

/«j^<r/j.— Mouth  parts  of,  331.  Of  beetle,  332.  Of  grasshopper 
serial  homology  of  these  parts,  333.  Mouth  parts  of  butterfly,  334. 
Of  bee,  335.  Digestive  apparatus  of  beetle,  336.  Crustacea,  mouth 
parts  of,  337. 

Mollusca. — Digestive  apparatus  of,  acephala,  338.  Gastropoda,  339. 
Radula,  cephalopoda,  340. 

Echinoderms.—ViTx.'sXxc'aXxw^  apparatus  of  echinus,  341. 

Cw/enterata.—Mediisa:,  342.  Lasso  cells,  343.  Polvps,  structure, 
344.     Modes  of  digestion, /?wost;(Z,  infusoria,  345.     Rhi'zopods,  346. 


CHAPTER   III. 

BLOOD    SYSTEM. 

SECTION    I. 

The  Blood. 

1.  Globules. — Red  globules,  347.  Structure  of,  348.  White  glob- 
ules (leucocytes),  blood  plates,  349. 

2.  /"/ajww —Coagulation  of  blood,  350.  Functions  of  Blood :  Of 
plasma,  of  red  globules,  351  ;  of  leucocytes,  352. 

Origin  0/  Blood.— (i)  Of  plasma,  (2)  of  leucocytes,  352.  (3)  Of  red 
globules,  353. 

Comparative  Morphology  of  Blood. — (i)  Mammalian  blood,  charac- 
teristic of,  (2)  oviparous  vertebrate  blood,  chaiacteristic  of.  354.  (3) 
Higher  invertebrate  blood,  characteristic  of,  (4)  coelenterate  blood, 
(5)  protozoa.     Embryonic  development  of  blood,  356. 


SECTION   II. 
Respiratory  Organs  0/  Vertebrates. 

Lungs  vs.  gills.  Lungs  of  man,  ■}<i'?>.  Structure,  359.  Mechanics  of 
breathing,  diaphragm,  361.  Relation  of  pleura  to  lungs,  362.  Costal 
respiration,  363.  Diaphragmatic  or  abdominal  lespiration,  364.  Cough- 
ing, laughing,  etc.,  365. 

Comparative  Morphology  of  Vertebrate  Respiration. — .\fammals, 
birds,  reptilt's,  2(>().  Tortoise,  amnhibians,  367.  Gill  ropiration,  f  sites, 
teleosts,  368.  Meclianics  of  gill-breathing,  369.  Variation  in  gills  of 
fishes,  sharks,  lampreys.  370.  Classification  of  fishes  by  respiratory  or- 
gans, 372.  Transition  from  gill  breathing  to  lung  breathing,  373. 
Classification  of  amphibians,  374. 


XVI    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

SECTION   III. 
Blood  Circulation —  Vertebrates. 

Circulation  in  man,  375.  General  course  of,  376.  Diagrams  illus- 
trating, 377,  378.  Structure  of  the  heart :  valves,  380.  Blood  vessels  : 
(i)  Arteries,  (2)  veins,  382  ;  {3)  capillaries,  383. 

Comparative  Alorphology  of  Blood  System  in  Vertebrates. — Mam- 
mals, birds,  reptiles  and  amphibians,  385.  Diagram  of  the  course,  386. 
Fishes,  diagram  of,  388. 

Bearing  of  these  Facts  on  Evolution. — (i)  Heart  structure,  {2)  aor- 
tic arches  and  their  relation  to  gill  arches,  390.  Diagram  of  origin  of 
aortic  arches  in  birds,  394.  In  mammals,  395.  Illustration  of  a  funda- 
mental law  of  evolution,  396. 

SECTION   IV. 
Morphology  of  Respiratory  and  Circulatory  System  in  Invertebrates. 

Introductory,  397.  Crustacea,  respiratory  organs  of,  mode  of  breath- 
ing, 399.  Circulation,  diagram  illustrating,  400.  Mollusca  :  Acephala, 
respiration,  401  ;  circulation,  402  ;  gastropods,  403  ;  cephalopoda,  404. 
Echinoderms,  404.  Ccelenterata,  protozoa,  406.  Insects,  why  passed 
over,  407.  Blood  system,  408.  Respiratory  system,  409.  Air  tubes, 
410.     Breathing,  411. 

SECTION   V. 

Lymphatic  or  Absorbent  System. 

General  description,  distribution,  411.  Structure,  function,  lym- 
phatic glands,  413.  Function,  comparative  morphology  of  lymphatic 
system,  414. 

CHAPTER    IV. 

KATABOLISM. 

SECTION    I. 
Introductory,  415.     Secretion   vs.  excretion,  416. 

SECTION    II. 

Function  of  Respiration. 

(i)  Chemistry  of  respiration.  418.  (2)  Purpose  of  combustion,  (3) 
the  fuels,  419.  (4)  How  is  force  created?  420.  Illustrative  diagram, 
422.  Place  of  the  combustion,  423.  Relation  of  plants  to  animals  in 
the  creation  of  animal  force,  424.  , 


CONTENTS.  XVll 

SECTION'    III. 
Kidneys  and  their  Function. 

Place  and  Form,  425.  E.\cretory  duct,  426.  Pelvis  of  kidney,  sec- 
tion, minute  structure,  427.  Function,  composition  of  urine,  429. 
Comparison  of  kidneys  and  lungs,  430.  Diagrams  illustrating  circula- 
ti  )n  of  C  and  O,  and  of  circulation  of  elements  of  organic  matter,  432. 
Comparative  Alorpholoi;}'  of  Kidneys  :  .Mammals,  birds,  reptiles,  433. 
Insects,  Crustacea,  mollusca,  434. 

SECTION    IV. 
The  Skin  and  its  Function. 

Function,  435.  Exhalation,  excretion,  structure  of  skin,  436.  Su- 
dorific glands,  437.     Lungs,  kidneys,  and  skin  compared,  438. 

Compa^-ative  Morphology  and  Physiology  of  Skin. — General  remarks, 
438.  Mammals,  birds,  reptiles,  amphibians,  ti.shes,  439.  Insects, 
crustaceans,  mollusca,  echinoderms,  cct-lenterates,  440. 

SECTION    V 

The  Liver  and  its  Function. 

Position  and  structure  of  liver,  440.  Its  four  systems  of  tubes,  441. 
Threefold  function,  442.  Glycogeny,  443.  Proof  of,  444.  Origin  of 
glycogen  threefold,  445.  The  use  of  glycogen  as  fuel,  447.  Diagram 
illustrating  the  process  of  change,  448.  Cause  of  diabetes,  449.  Com- 
parative morphology  of  liver,  450. 

CHAPTER    V. 

TEGUMENTARY   ORGANS — SKIN    STRICTURES. 

SECTION    I. 

Vertebrates. 

Structure  of  vertebrate  skin,  various  changes  in  epidermis,  452. 
Importance  in  classification,  hair,  453  Nails,  claws,  454.  Hoofs, 
horns,  455.  Feathers,  456.  Structure  of  featliers,  adaptation  to  flight, 
457.  Mode  of  formation  of  feathers,  gradation  to  hairs,  458.  Scales, 
459.  Classification  of  fishes  by  scales,  460.  Reptile  scales,  rattle  of 
rattlesnake,  461.  Turtle  shell,  462.  Mammalian  shell.  Endo.skeleton 
and  exoskeleton,  463. 

SECTION    II. 

Invertebrates. 

Insect  shell,  463.  Higher  crustaceans,  464.  Mollusca  :  Bivalves, 
growth  of  shell,  464  ;  gastropod,  cephalopod,  classification  of  mollusca 
by  shell,  465.     Echinoderms,  structure  of  the  shell,  466. 

Corals,  466.     Structure  of,  467.     Structure  and  mode  of  formation 
of  the  theca,  46S.     Sponge,  skeleton  of,  469.     Rhizopod  shell,  470. 
2 


XVill    PHYSIOLOGY  AND  MORPHOLOGY  OF  ANIMALS. 
CHAPTER   VI. 

GEOGRAPHICAL   DISTRIBUTION   OF   ORGANISMS. 

Definition  of  fauna  and  flora,  471.  Illustration  of  harmonic  rela- 
tions of  faunas,  botanical  temperature  reL:ions,  472.  Zoological  tem- 
perature regions,  completer  definition  of  temperature  regions,  range  of 
species,  genera,  etc.,  474.  Mode  of  grading  of  contiguous  ranges,  475. 
Effect  of  barriers,  475. 

Continental  iaunal  regions,  species  usually  distinct,  477.  Excep- 
tions, 478.  Subdivisions  of  continental  faunas,  479.  Special  cases : 
(i)  Australia,  479;  (2)  Madagascar,  480;  (3)  Galapagos,  481.  River 
mussels,  481. 

Mai'ine  Faunas. — Temperature  regions  of  east  coast  of  United 
States,  481.  Shore  faunas,  pelagic  fauna,  abyssal  faunas,  special  cases, 
482. 

Primary  Division  of  Land  Faunas. — Wallace's  divisions,  483. 
Schedule  of  regions  and  provinces,  subdivisions  of  the  Nearctic,  484. 

Theories  of  the  Origin  of  Distribution  of  Organisms. — Old  theory, 
485.  New  theory,  487.  Examples  of  explanation  by  new  theory  : 
(l)  Alpine  species,  (2)  Australia,  4S8  ;  (3)  Africa,  489.  Islands,  kinds 
of,  (4)  Madagascar,  490  ;  (5)  British  Isles,  (6)  California  coast  islands, 
491  ;  (7)  oceanic  islands,  492. 


OUTLINES  OF  COMPARATIVE  PHYSIOLOGY 
AND  MORPHOLOGY  OF  ANIMALS. 


INTRODUCTORY. 

SOME   GENERAL    PRINCIPLES. 
SECTION    I. 

RELATIONS    OF     THE     THREE     KINGDOMS    OF     NATURE     TO 
ONE    ANOTHER. 

Nature  is  primarily  divided  into  two  l<ingdoms, 
the  living  and  the  nonliving.  The  living  kingdom  is 
subdivided  into  the  animal  and  the  plant  kingdoms, 
thus  : 

_  .  .  \  3.  Animal. 

Living -        „, 

*  /  2.  Plant. 

Nonliving 1 .  Mineral. 

We  may,  indeed,  divide  Nature  into  three  kingdoms 
— mineral,  plant,  and  animal — as  seen  above;  but,  if  so, 
it  is  necessary  to  remember  that  the  gap  between  the 
organized  and  the  unorganized,  the  living  and  the  non- 
living, is  far  greater  than  between  the  two  divisions  of 
the  living  kingdom,  i.  e.,  between  plants  and  animals. 
The  study  of  unorganized  or  nonliving  Nature  belongs 
to  physics  and  chemistry  ;  the  study  of  organized  or 
living  Nature  belongs  to  biology. 

Living  vs.  Nonliving. — Besides  the  essential  proper- 
ties belonging  to  all  matter,  living  things  have  certain 

I 


2        niVSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 

additional  and  distinctive  properties.  These  are  all 
connected  with  the  endowment  of  life.  But  we  know 
nothing  of  life  except  by  its  properties.  The  same, 
however,  is  true  of  all  other  forces  of  Nature.  We 
know  not  what  they  are,  but  only  what  they  do,  how 
they  behave.  Life,  like  all  other  forces  of  Nature,  is  a 
specific  form  of  energy  c\\7i.x'6.c:\.&x\zit'\  by  a  peculiar  group 
of  phenomena,  the  subject-matter  of  a  distinct  science 
— biology.  What,  then,  are  the  peculiar  phenomena 
of  life  ? 

1.  Organization. — All  living  things  are  organized — 
i.  e.,  consist  of  different  parts  having  different  functions 
and  all  co-operating  for  one  given  end,  the  conserva- 
tion of  the  organism.  It  may  at  first  seem  that  by  this 
definition  a  machine,  say  a  steam  engine,  is  living;  but 
there  is  this  essential  difference:  the  steam  engine  re- 
quires an  external  force  determining  the  co-operation, 
while  in  living  things  the  co-operation  is  self-determined. 

2.  Cellular  Structure. — All  living  things  are  wholly 
made  up  of  cells,  as  completely  as  a  brick  building  is 
made  up  of  bricks.  Now  there  is  an  unorganized  cellular 
structure  also — such,  for  example,  as  in  foam,  in  soap- 
suds, in  vesicular  lava,  etc.  But  there  is  again  this  es- 
sential difference  :  in  unorganized  cellular  structure 
there  is  simply  a  homogeneous  mass  full  of  hollow 
spaces,  while,  on  the  contrary,  in  organized  cellular 
structure  each  cell  is  a  living  entity — the  organized  body 
is  a  community  of  living  cells. 

3.  Growth. — All  living  things  grow.  But  do  not 
nonliving  things  grow  also  ?  Take  a  saturated  solution 
of  sugar,  or  alum,  or  salt.  By  evaporation  crystals  are 
formed.  Small  at  first,  they  grow  from  hour  to  hour, 
from  day  to  day,  until  they  become  as  large  as  a  walnut 
or  even  as  one's  fist.  But  again,  we  find  essential  dif- 
ferences in  the  kind  of  growth  :   i.  The  growth  of  the 


RELATIONS    OK    THE     ITIREE    KINCJDOMS. 


3 


crystal  is  by  additions  to  the  outside,  layer  by  layer; 
the  growth  of  the  living  thing  is  by  material  taken  into 
every  part  of  each  cell — interstitial  growth.  2.  Again, 
the  growth  of  the  crystal  is  by  materials  exactly  like 
itself  already  existing  in  the  liquid,  not  made  by  the 
crystal  ;  whereas  the  living  thing  makes  the  material  of 
its  growth  ;  it  manufactures  material  like  itself  out  of  mate- 
rial wholly  different  from  itself,  and  then  uses  it  for  groicth 
— growth  by  assimilation. 

4.  Life  History. — All  living  things  pass  through  a 
regular  cycle  of  changes  determined  by  forces  within 
itself.  They  are  born,  increase,  culminate,  decay,  and 
finally  die.  No  such  cycle  of  changes  is  observed  in 
nonliving  things.  Whatever  changes  they  undergo  are 
accidental  and  the  result  of  external  causes. 

5.  Reproduction. — As  living  things  have  a  definite 
term  of  existence,  they  must  reproduce  their  kind; 
otherwise  the  organic  kingdom  would  speedily  pass 
out  of  existence.  This  also  is  characteristic  of  living 
things. 

6.  Waste  and  Supply — Metabolism.— Continual 
change  of  the  material  composing  the  body  in  every 
part  is  very  characteristic  of  living  things.  The  living 
body  has  been  compared  to  a  whirlpool  :  the  matter  is 
continually  changing  w^hile  the  form  remains.  In  the 
living  body  the  change  is  in  some  sense  self-determined. 
This  ceaseless  internal  change  by  waste  and  supply  is 
called  metabolism.  It  is  rapid  in  proportion  as  the  life 
is  high. 

We  are  concerned  here  only  with  living  things.  All 
dead  things  and  the  sciences  which  concern  them  are 
therefore  put  aside;  but  we  must  still  limit  our  field. 
We  are  not  concerned  with  all  living  things,  but  only 
with  animals.  We  must  therefore  distinguish  between 
animals  and  plants. 


PHYSIOLO(;V    AND    MORPHOLOGY    OF    ANIMALS. 


Fig.  r. — Diagfram  represent- 
ing the  differentiation  of 
animals  and  plants  from 
Protista. 


Animals  vs.  Platits. — The  distinctions  between  ani- 
mals and  plants  are  apparently  so  obvious  that  it  may 
seem  useless  to  draw  attention  to  them,  but  it  is  so  only 
to  careless  view  and  in  compar- 
ing the  higher  members  of  the 
two  kingdoms.  As  we  descend 
in  the  scale  the  two  kingdoms 
approach  more  and  more,  until 
they  absolutely  come  together — 
in  other  words,  the  living  king- 
dom in  its  lowest  members 
consists  of  beings  which  are 
both  animals  and  plants,  or  else 
neither.  They  are  living  things 
7<.nthoiit  further  qualification.  In 
the  present  state  of  our  knowl- 
edge they  may  be  claimed  by  either  botany  or  zoology, 
and  it  is  proposed  to  call  them  Protista,  or  lowest  living 
beings.  From  these  lowest  beginnings  the  two  kingdoms 
separate  more  and  more  as  we  rise.  Where  they  first 
separate  we  call  them  Protozoa  (first  or  lowest  animals) 
and  Protophyta  (first  or  lowest  plants).  Then  follow  the 
more  distinctive  animals  and  plants,  but  the  animals  rise 
the  higher,  as  in  Fig.  i. 

It  is  not  so  easy,  then,  to  define  the  limits  of  the 
animal  kingdom.  Popularly,  perhaps,  animals  would  be 
defined  as  beings  capable  of  motion;  but  this  will  not  do, 
for  many  plants  also  move  under  stimulus,  as,  for  exam- 
ple, the  sensitive  plant.  Or  perhaps  locomotion  is  sup- 
posed to  be  characteristic  of  animals ;  but  this  also 
fails,  because  many  of  the  lower  plants  and  the  embryos 
of  some  higher  ones  move  about  with  such  rapidity 
that  they  can  hardly  be  observed  carefully  under  the 
microscope.  On  the  other  hand,  many  animals  some- 
what high  in  the   scale,    such    as    ovsters,    corals,    etc., 


RELATIONS    OF    THE    THREE    KINGDOMS. 


are  incapable  of  locomotion.     What,  then,  are  the  dif- 
ferences ? 

1.  Sensation  and  Volition. — Doubtless  conscious 
sensation  and  voluntary  motion  are  characteristic  of 
animals,  but  the  difficulty  is  in  applying  the  test.  We 
conclude,  and  probably  rightly,  that  the  motions  of 
plants  are  unconscious  and  involuntary.  In  the  case 
of  any  motion,  if  we  could  be  certain  that  it  was  at- 
tended with  consciousness,  we  would  rightly  conclude 
that  the  moving  thing  was  an  animal.  But  how  are  we 
to  know?  Very  many  of  the  motions  of  animals,  and 
even  motions  within  our  own  bodies,  such  as  motions  of 
the  heart,  stomach,  etc.,  are  wholly  unconscious  and  in- 
voluntary. 

2.  Nature  of  the  Food  and  the  Relation  to  the 
Mineral  Kingdom. — Plants  feed  on  the  mineral  king- 
dom directly  ;  animals  feed  on  plants  or  on  one  another. 
The  food  of  plants  is  mineral  matter  ;  the  food  of  animals 
is  organic  matter  made  by  plants.  More  specifically, 
the  food  of  plants  is  COg.HgO  and  XH3.  These  purely 
mineral  matters  are  taken 

and     decomposed.       Some  a*'""^/'?*''-? 

parts  are  thrown  away,  and 
the  remainder  are  made 
to  combine  into  new  sub- 
stances not  made  elsewhere 
— i.  e.,  organic  substances, 
such  as  starch,  sugar,  cellu- 
lose, and  especially  proto- 
plasm. Thus  all  the  or- 
ganic substance  in  the  world 
is  created  by  plants,  under 
the  influence  of  sunlight.  Animals,  so  far  from  creating, 
are  constantly  destroying  organic  matter  and  resolving 
it  into  its  original  components.     Thus  the  relations  of 


<^f^r;;^i 


Fig.  2. — Diagfrain  illustrating:  the 
circulation  of  carbon  and  oxygen. 


(',         I'HVSIULOGY    AND    MOUrilOLOGV    OF    ANliVlALS, 

the  two  living  kingdoms  to  the  mineral  kingdom  aie  the 
converse  of  one  another:  plants  making  organic  matter 
from  minerals,  and  animals  destroying  organic  matter 
and  returning  it  again  to  the  mineral  kingdom.  Limit- 
ing our  view  now  to  one  of  these  mineral  plant-foods, 
COo-.  plants  decompose  COo,  returning  the  oxygen  to 
the  air  and  retaining  the  carbon  to  make  with  other  ele- 
ments organic  matter;  animals  contrarily  take  carbon 
from  plants  in  tlTe  form  of  organic  matter  of  food,  and 
oxygen  from  the  air  in  respiration,  combine  these,  and 
restore  them  to  the  air  as  COg.  Thus  there  is  a  con- 
tinual circulation  of  carbon  and  oxygen  between  these 
three  kingdoms,  as  shown  in  the  diagram.  Fig.  2.  Thus 
the  plant  kingdom  is  a  necessary  intermediary  between 
the  mineral  and  the  animal  kingdoms. 

This  is  probably  the  best  and  most  philosophical  dis- 
tinction between  the  two  kingdoms  ;  but  even  in  this 
there  is  a  gradation  as  we  go  down  the  scale  of  life.  In 
any  case  we  want  an  easier  and  more  practical  test.  We 
find  this  in 

3.  The  Possession  of  a  Stomach. — Animals  have 
stomachs,  plants  have  not.  This  is  really  a  philosoph- 
ical distinction,  because  it  is  closely  connected  with  the 
nature  of  the  food.  The  food  of  plants  is  mineral. 
This  mineral  food  exists  in  solution  in  water,  or  else  in  a 
gaseous  state  in  the  atmosphere..  It  is  therefore  already 
in  condition  to  be  at  once  absorbed  without  further 
preparation.  It  is  thus  absorbed  by  the  surface  of  the 
roots  and  of  the  leaves — external  absorption.  But  the 
food  of  animals,  being  organic  matter,  is  usually  in  a 
more  or  less  solid  condition,  and  can  not  be  absorbed 
until  it  is  dissolved;  and  this  requires  time.  Therefore 
animals  must  have  a  reservoir  in  which  the  food  is 
stored  until  it  is  reduced  to  a  liquid  condition  fit  for 
absorption — internal  absorption.      This    reservoir   is    the 


RELATIONS   OF   THE    THREE    KINGDOMS.  7 

Stomach.  All  animals  therefore  must  have  a  stomach. 
In  the  very  lowest  animals,  however,  this  organ  is  ex- 
temporized for  use  when  wanted.  A  single  cell,  an  al- 
most microscopic  spherule  of  gelatinous  protoplasm, 
meets  its  food,  flows  around  it,  takes  it  in,  and  di- 
gests it  (Fig.  156,  p.  240). 

Again,  the  food  of  plants  is  everyic/iere  present.  In 
the  form  of  solution  it  bathes  the  roots,  in  the  form  of 
gases  it  bathes  the  surface  of  the  leaves.  The  food- 
taking  is  passive.  Animals  seek  their  food,  and  usually, 
but  not  always,  move  about  to  gather  it.  This,  again,  re- 
quires a  reservoir  in  which  to  keep  it  while  it  is  being 
prepared  for  absorption.  Both  the  kind  of  food  and  the 
mode  of  taking  it  require  a  stomach.  All  these  four — 
viz.,  kind  0/  food,  the  possession  of  a  stomach,  power  of 
voluntary  motion,  and  the  seeking  of  food  or  desire — are 
closely  connected  with  one  another;  and  the  kind  of 
food — i.  e.,  organic  matter — is  the  basis  of  all.  For  this 
necessitates  appetite,  therefore  seeking  of  food,  and  this 
locomotion  and  a  reservoir  to  store ;  therefore  all  are 
characteristic  of  animals. 

4.  Waste  and  Supply. — Continual  internal  change, 
as  already  seen,  is  coextensive  with  life.  But  this  internal 
change  is  far  more  rapid  in  animals  than  in  plants.  In 
plants,  supply  is  always  in  excess  of  waste,  and  therefore 
plants  grow  as  long  as  they  live.  In  animals,  on  the 
contrary,  in  early  life  supply  is  in  excess  of  waste,  in 
maturity  they  balance  and  there  is  no  growth,  in  age 
waste  is  in  excess.  Again,  as  we  shall  see  hereafter,  the 
whole  of  animal  force  is  derived  from  waste,  while  in 
plants  only  a  small  part  is  thus  derived,  the  rest  being 
derived  from  sunlight. 

We  have  now  delimited  our  field  of  study  from  other 
kingdoms  of  Nature — it  is  animals  ;  and  our  science  from 
other    departments    of    science — it  is    zoology.     But  the 


8        PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

field  of  zoology  is  far  too  extensive;  we  can  occupy  but 
a  small  part. 


SECTION    II. 

DEFINITION    OF     ZOOLOGY    AND    THE     SCOPE    AND     LIMITS 
OF    THIS    COURSE. 

Zoology  is  the  science  of  animals.  It  embraces 
every  scientific  question  that  may  be  raised  concerning 
animals — their  form,  structure,  functions,  habits,  their 
affinities,  their  distribution  in  time  geologically  and  in 
space  geographically.  The  subject  of  this  course  is  only 
a  part  of  zoology. 

The  most  fundamental  divisions  of  zoology  may  be 
clearly  brought  out  by  considering  an  animal  in  differ- 
ent ways.  We  may  study  the  external  form  of  the 
whole  and  of  each  part;  then  cut  into  and  dissect  and 
determine  the  form  and  structure  of  the  internal  organs 
as  far  as  the  naked  eye  can  see.  Then  with  the  micro- 
scope we  determine  the  minute  structure  of  every  tis- 
sue and  organ.  All  of  this  is  anato?n}\  although  the 
naked-eye  anatomy  is  often  called  morphology,  and  the 
minute  anatomy  histology.  All  this  can  best  be  studied 
in  the  dead  animal. 

Or,  again,  we  may  study  the  functions  of  the  living 
animal — i.  e.,  the  work  that  each  part  or  organ  performs, 
and  the  manner  in  which  they  all  co-operate  for  the 
common  life  and  happiness  of  the  animal.  This  is 
called  physiology,  and  can  be  studied  only  in  the  living 
animal. 

But,  again,  we  may  study  not  only  the  living  mature 
animal,  but  the  living  growing  animal.  Commencing 
with  the  yet,  apparently,  unorganized  t%%,  we  may  trace 


DEFINITION    OF    ZOOLOGY.  g 

the  gradual  development  of  each  part  and  of  the  func- 
tion which  it  performs  to  its  completion  in  the  mature 
condition.     This  is  embryology. 

Now,  if  there  were  but  one  kind  of  animal  in  the 
world,  the  study  of  that  species  from  these  three  points  of 
view  would  still  be  inexhaustible.  But  the  field  of  study 
becomes  far  wider  when  we  remember  that  there  are  an 
almost  infinite  variety  of  kinds,  of  every  degree  of  com- 
plexity of  organization,  and  that  it  is  only  or  chiefly  by 
comparison  of  these  kinds  with  one  another  that  gen- 
eral law's  are  reached.  Thus  each  of  these  departments 
must  be  made  comparative  before  it  can  become  truly  sci- 
entific. Thus,  then,  the  three  fundamental  departments 
of  zoology  thus  far  found  are  (i)  comparative  anatomy, 
(2)  comparative  physiology,  and  (3)  comparative  embryology. 

But  the  number  and  variety  of  animals  is  so  great, 
the  material  of  science  is  so  immense,  that  it  is  wholly 
unmanageable  and  even  bewildering  unless  arranged 
and  classified  in  an  orderly  way.  Therefore  animals 
are  divided  and  subdivided  into  groups,  according  to 
their  affinities  or  their  differences  and  resemblances ; 
those  of  the  larger  groups  united  with  one  another  by 
the  most  general  characters,  and  separated  from  other 
groups  by  the  most  profound  differences,  the  smaller 
groups  being  united  by  smaller  but  more  numerous  re- 
semblances, and  separated  by  less  profound  differences. 
This  is  called  (4)  taxonomy,  or  classification. 

But  such  orderly  arrangement  can  not  be  made  with- 
out extensive  knowledge  of  animals  in  all  parts  of  the 
world,  such  as  no  one  man  can  individually  acquire. 
Therefore  it  is  necessary  to  describe  all  the  kinds  of  ani- 
mals, each  in  its  proper  place  in  the  orderly  arrange- 
ment. But  this  can  be  done  only  by  the  co-operation  of 
all  zoologists  in  all  parts  of  the  world,  and  the  publica- 
tion of  these  results,  so  that   each  can   use  the  work  of 


lO      PHYSIOLOGV    AND    MORPHOLOGV    OF    ANIMALS, 

all.  This  is  called  (5)  descriptive  zoology.  This  consti- 
tutes, along  with  taxonomy,  systematic  zoology. 

But  the  scope  of  the  subject  is  not  yet  exhausted. 
The  earth  has  been  inhabited  for  millions  of  years,  and 
the  animal  forms  have  been  changing  during  all  this  time 
according  to  certain  definite  laws.  The  study  of  extinct 
forms,  and  especially  the  laws  of  succession  of  forms, 
constitutes  another  department,  called  {6)  palceozoology. 

Finally,  the  animals  inhabiting  different  parts  of  the 
earth  are  very  different  from  one  another.  The  causes 
of  these  differences  and  their  laws  constitute  another 
absorbingly  interesting  department,  for  which  no  uni- 
versally accepted  name  has  yet  been  proposed.  It  has 
been  called  chorology,  but  is  usually  spoken  of  as  geo- 
graphical distribution  of  animals,  or  (7)  geographical  zoology. 

Thus,  then,  the  main  departments  of  zoology  are: 

1.  Comparative  anatomy  or  morphology. 

2.  Comparative  physiology. 

3.  Comparative  embryology. 

4.  Taxonomy,  or  classification 

5.  Descriptive  zoology 

6.  Falfeozoology. 

7.  Geographical  zoology. 

Each  one  of  these  departments  constitutes  a  field  of 
study  sufficient  to  occupy  the  lifetime  of  any  one.  It 
is  evident,  then,  that  we  can  not  take  up  all  these  with 
equal  fullness.  Instruction  spread  over  so  wide  a  field 
must  be  far  too  meager.  We  select,  then,  as  our  cen- 
tral subject  the  second  one,  comparative  physiology  of  ani- 
mals. For  this,  which  deals  with  the  phenomena  of  ani- 
mal life,  is  certainly  the  department  to  which  all  others 
are  tributary.  But  it  is  impossible  to  understand  function 
without  a  knowledge  of  structure,  with  which  it  is  as- 
sociated. Therefore  our  subject  will  be  physiology,  and 
so  much  anatomy  or  morphology  as  is  necessary  to  ex- 


(.  systematic. 


GENERAL   STRUCTURE    OF    ANIMALS.  j  i 

plain  the  physiology,  and  often  something  more  when 
the  morphology  has  an  important  bearing  on  classifica- 
tion, and  especially  when  it  has  an  important  bearing 
on  the  question  of  evolution.  Embryology  we  will  touch 
on  only  when  it  bears  in  an  important  way  on  the  same 
two  subjects.  Classification  we  shall  not  touch  at  all 
except  in  the  indirect  way  explained  above.  Some 
scheme  of  classification,  however,  we  must,  of  course, 
assume  as  the  necessary  condition  of  study,  for  we  can 
deal  with  animals  only  ///  groups.  But  this  we  put  off 
until  we  must  have  it.  Lastly,  if  we  find  time  we  will 
devote  some  pages  to  the  laws  of  geographical  distri- 
bution of  animals,  as  this  has  a  most  important  bearing 
on  the  question  of  evolution. 

So  much  to  define  the  scope  and  limits  of  our  course. 
It  is  limited  (i)  to  the  science  of  life,  biology  :  (2)  to 
the  science  of  animal  life,  zoology ;  (3)  in  zoology  it  is 
limited  to  comparative  physiology  mainly,  but  not  entirely, 
for  function  is  indissolubly  united  with  structure  and 
form.  It  may  therefore  be  called  comparative  physiology 
and  morphology. 

SECTION    III. 

GENERAL    STRUCTURE    OF    ANIMALS. 

We  have  already  said  that  all  or- 
ganisms are  composed  of  living  cells. 
A  living  cell  consists  of  three  parts, 
viz.,  (i)  a  mass  of  semiliquid  proto- 
plasm, usually  granulated,  (2)  a  nu- 
cleus of  more  solid  matter,  and  (3) 
a  thin  delimiting  membrane  (Fig.  3). 
Cellular  structure  is  coextensive  with  life,  but  the  cells 
of  animals  differ  considerably  from  those  of  plants,  and 
are  far  less  distinct  for  the'following  reasons: 


12      PHYSTOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

1.  Size. — Cells  are  nearly  always  microscopic  in 
size,  but  are  far  more  minute  in  animals  than  in  plants. 

2.  Softness. — In  plants  each  cell  is  incased  in  a 
firm  shell  of  cellulose,  so  that  in  thin  sections,  such  as 
are  used  in  microscopic  examination,  they  retain  their 
form  perfectly;  while  in  animals,  unless  specially  pre- 
pared, they  collapse  and  lose  their  cellular  appearance. 

3.  Transparency. — The  cells  of  animals,  unless 
specially  prepared  by  staining,  are  so  transparent  that 
their  outlines  are  often  detectable  with  difficulty.  This 
is  far  less  true  in  plants. 

4.  More  highly  Differentiated. — But  with  proper 
care  all  these  difficulties  may  be  overcome.  The  real 
greatest  difficulty  is  the  differentiation  of  cell  form, 
which  is  much  greater  in  animals  than  in  plants.  Cells 
take  on  different  forms  in  order  to  perform  different 
functions.  Therefore  as  functions  increase  in  number 
and  become  more  perfect  the  cells,  take  on  more  numer- 
ous forms,  and  the  forms  differ  more  and  more  from  one 
another,  and  all  from  simple  undifferentiated  cells.  Now 
functions  are  more  numerous  and  higher  in  animals  than 
in  plants,  and  therefore  the  structure  of  animals,  espe- 
cially the  higher  animals,  differs  greatly  from  simple  cellu- 
lar structure  ;  so  that  in  the  mature  condition  of  the  high- 
er animals  the  simple  cellular  structure  may  be  entirely 
lost.  The  universal  cellular  structure  of  animals  there- 
fore is  best  seen  in  the  lowest  animals  and  in  the  embry- 
onic condition  of  the  higher. 

TISSUES. 

A  tissue  may  be  defined  as  an  aggregate  of  cells  of 
the  same  form  and  having  the  same  function^  but  differing 
in  form  and  function  from  the  cells  of  other  aggrega- 
tions; different  tissues  therefore  are  different  styles  of  cell 
structure,  each  adapted  to  a  peculiar  function.     The  ani- 


GENERAL  STRUCTURE  OF  ANIMALS. 


13 


mal  body  is  made  up  of  cells  as  completely  as  a  brick 
building  is  made  up  of  bricks;  but  as  we  may  have 
different  kinds  of  brickwork  adapted  for  various  pur- 
poses, so  we  have  various  kinds  of  cell  work,  and  these 
constitute  the  different  tissues.  I'he  kinds  of  tissues  are 
more  numerous  and  more  varied  as  the  functions  are 
more  numerous  and  higher,  and  consequently  as  we  rise 
in  the  scale  of  organization.  Therefore  they  are  more 
numerous  and  varied  in  animals  than  in  plants,  and  in 
the  higher  than  in  the  lower  animals.  It  would  be  use- 
less and  confusing  to  speak  of  all  the  kinds  of  tissues 
treated  in  special  works  on  his- 
tology. We  shall  treat  of  six 
general  kinds,  although  some  of 
these  will  be  subdivided. 

I.  Connective. — This  con- 
sists of  transparent  white  inter- 
laced fibers  or  bands  of  fibers 
running  in  all  directions,  forming 
sometimes  a  loose  mesh,  some- 
times a  closely  felted  structure,  in  fig.  4.— Connective  tissue. 
which  are  found  scattered  spindle- 
shaped  nucleated  cells  with  continuing  branching  fibers 
(Fig.  4).  It  is  called  connective  because  it  penetrates 
and  supports — it  connects,  and  yet  separates,  all  the 
other  tissues  and  organs  of  the  body,  forming  a  sort  of 
universal  warp  in  which  all  other  tissues  are  woven  as  a 
woof  determining  the  pattern  of  the  fabric;  so  that  if  all 
other  tissues  could  be  picked  out  and  removed  and  this 
one  only  remained,  the  whole  form  of  the  body  with  all 
its  organs  would  be  retained  in  the  connective  tissue. 

Examples. — In  skinning  the  dead  body  of  an  animal, 
as  we  pull  the  skin  from  the  underlying  flesh  we  observe 
a  white  shining  mesh  connecting  them.  This  is  divided 
with  the  knife  in  the  act  of  skinning.     This  is  the  subcu- 


H 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS 


taneous  connective.  It  is  so  loose  a  mesh  that  water  or 
air  may  be  pumped  in  so  as  to  swell  up  the  whole  body. 
General  dropsy,  in  fact,  arises  from  accumulation  of 
water  in  this  tissue.  When  the  skin  is  removed  we  may 
dissect  between  and  separate  the  several  muscles.  In 
doing  so  we  cut  the  connective  between  the  muscles. 
The  same  tissue  in  a  denser  form  constitutes  the  invest- 
ing membrane  of  the  muscle.  If  we  cut  into  the  muscle 
we  find  that  every  fiber  is  invested  by  the  membranous 
form,  and  separated  from  its  neighbor  fiber  by  the  mesh 
form  of  the  same.  In  brief,  we  may  say  that  this 
tissue  in  its  denser  form  invests  every  fiber,  and  in  its 
loose  form  separates  and  yet  connects  the  fibers  ;  then 
it  emerges  on  the  surface  of  the  muscle  in  denser  form, 
investing  and  individuating  it ; 
and  finally  in  loose  form  lies  be- 
tween the  muscles,  separating  and 
yet  connecting  them.  The  same 
is  true  of  all  the  organs  of  the 
body. 

Varieties. — The  loose,  meshlike 
form  is  called  areolar  tissue,  or 
sometimes  cellular  tissue.  The 
denser  form  is  called  fibrous  tissue. 
By  a  little  stretch  of  our  defini- 
tion, the  skin,  which  consists  of 
closely  felted,  interlacing  fibers, 
may  be  regarded  as  an  extreme 
variety  of  connective.  It  is  called 
dermoid  tissue.  Scattered  about 
among  the  interlacing  fibers,  es- 
pecially of  the  areolar  variety,  are  found  nucleated, 
spindle-shaped  cells  with  branching  ^h^x^ —connective-tissue 
cells.  Also,  the  same  variety  is  usually  the  place  of  de- 
posit of    so-called  fat  cells,  which    accumulate   often   in 


Fig.  5. — Connective  tissue 
with  fat  cells. 


GENERAL  STRUCTURE  OF  ANIMALS. 


i; 


Fig.  6. — Structure  of  cartilage. 


large  quantities  (Fig.  5).     This  is  sometimes  spoken  of 
as  adipose  tissue,  but  it  is  not  properly  a  tissue  at  all. 

2.  Cartilage. — This  will  be  easily  recognized  under 
its  popular  XidiVa^t,  gristle.  \\%  firmness  din<\  yet  elasticity, 
its  white  translucency,  its  smooth  homogeneous  surface 
when  cut,  are  familiar  and 
characteristic  properties.  If 
a  thin  section  be  placed  under 
the  microscope,  it  is  at  once 
seen  to  consist  of  innumer- 
able nucleated  cells  lying  in 
a  structureless,  semitranspar- 
ent,  hyaline  mass  (Fig.  6).  The 
cells  are  in  clusters,  evidently 
formed  by  cell  division. 

Cartilage  is  the  tissue  used 
in  the  animal  body  whenever  a  moderate  degree  of  firm- 
ness combined  with  elasticity  is  required.  It  therefore 
caps  the  ends  of  the  bones  at  the  joints.  The  anterior 
portions  of  the  ribs  are  cartilage,  so  as  to  yield  to  respi- 
ratory motions.  The  external  ear  consists  of  cartilage, 
so  as  to  retain  its  form  and  yet  to  be  not  liable  to  break. 
The  tip  of  the  nose  is  of  the  same  substance,  and  for  the 
same  reason. 

Varieties. — In  higher  animals  there  are  two  varieties, 
viz.,  permanent  cartilage,  such  as  all  those  alreadv 
mentioned,  and  temporary  cartilage,  which  afterward 
becomes  bone.  Hence  cartilage  is  very  abundant  in 
young  animals.  But  the  difference  between  these  varie- 
ties is  too  unimportant  to  detain  us. 

3.  Bony  Tissue. — Bone  is  the  hardest  tissue  in  the 
body  and  is  used  wherever  rigidity  is  required.  It  is 
therefore  in  higher  animals  the  material  of  the  skeleton. 
It  consists  of  an  organic  tissue,  a  kind  of  connective, 
hardened  by  a  deposit  in  it  of  mineral  matter,  chiefly 


l6      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

lime  phosphate.  These  two  parts  may  be  easily  sepa- 
rated. If  bone  be  thoroughly  burned,  the  organic  mat- 
ter is  consumed  and  the  white  lime  phosphate  remains, 
retaining  the  form,  the  structure,  and  the  stiffness  of 
the  bone.  On  the  other  hand,  if  a  bone  be  immersed  in 
a  weak  acid — HCl — for  several  days,  the  lime  phosphate 
will  be  dissolved,  leaving  the  organic  tissue,  also  re- 
taining the  form  and  structure  but  not  the  stiffness  of 
the  bone.     It  may  now  be  tied  in  a  knot. 

Structure. — Looking  closely  on   the  surface  of  bone 
one  can  see  with  the  naked  eye  long   channels,  and  on 


Fig.  7.— Structure  of  bone :   A,  lonp:itudinal ;  B,  cross-section ;  h.  Haver- 
sian canals. 


cross-section  pores  like  those  seen  in  wood.  In  this  re- 
spect it  differs  from  ivory,  which  has  no  such  pores. 
These  are  the  Haversian  canals  (Fig.  7,  Ji).  They  are,  in 
fact,  blood  vessels  of  the  bone.  This  is  as  much  as  can 
be  seen  with  the  naked  eye.  Under  the  microscope,  in 
addition,  we  see  that  the  bony  matter  is  arranged  in  con- 
centric circles  or  cylinders  about  the  canals,  and  that 


GENERAL    STRUCTURE    OF    ANIMALS. 


17 


scattered  numerously  through  the  bony  matter  are  black 
cavities.  These  are  the  lacuiuc.  Running  from  these  in 
all  directions,  like  the  legs  of  an  insect,  are  very  slender 
tubes  connecting  the  lacunae  with  one  another  and  with 
the  canals.  These  are  the  canalicles  (canaliculi).  The 
red  blood  circulates  freely  through  the  canals,  but  the 
blood  globules  are  too  large  to  pass  the 
canalicles.  Therefore  only  colorless  blood 
plasma  reaches  the  lacunns. 

As  already  said,  bone  is  the  material 
of  the  skeleton  of  higher  vertebrates,  but 
as  we  pass  backward  in  the  embryonic 
series,  or  down  in  the  animal  series,  car- 
tilage replaces  bone.  In  other  words, 
cartilage  is  the  embryonic  or  imperfect 
form  of  bone. 

Origin. — Bone  comes  usually  from  car- 
tilage by  deposit  of  mineral  matter  ;  but 
bone  may  also  be  formed  by  deposit  of 
mineral  matter  in  other  tissues,  as  in 
fibrous  membranes  and  in  skin.  So  we 
have  cartilage  bones,  membrane  bones, 
and  skin  bones. 

Varieties. — Dentine  is  a  denser  kind  of  bone  in  which 
the  canals  are  wanting.  This  is  the  principal  material 
of  teeth.  Ivory  is  the  finest  example.  Enamel  is  a  still 
denser  variety  of  bone  which  covers  the  teeth  of  higher 
vertebrates  (Fig.  8). 

We  must  bear  in  mind  the  distinction  between  a 
bone  as  an  organ  and  l>one  as  a  tissue  or  material  of 
which  the  organ  is  composed. 

4.  Muscle  Tissue. — We  must  also  distinguish  here 
between  a  muscle  as  an  organ  and  muscle  as  a  tissue.  A 
muscle  is  an  organ  consisting  of  several  tissues — for  ex- 
ample, connective,  nervous  tissue,  etc. — but  its  charac- 


FiG.  8. 
Section  of  tooth. 


1 8      PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


teristic  tissue  is  the  muscular.  The  characteristic  prop- 
erty of  this  tissue  is  contractility  under  stimulus  of  any 
kind,  but,  normally,  under  the  stimulus  of  nerve  force. 

Structure. — To  the  naked  eye  muscle  tissue  consists 
of  bundles  or  fascicles  of  fibers  lying  parallel  to  one  an- 
other; but  under  the  microscope  each  fascicle  is  seen  to 
be  composed  of  a  multitude  of  cylindrical  fibers  trans- 
versely striated,  and  under  favorable 
conditions  these  again  are  separable  C 

into  still  finer  fibrilla3.      Each  fiber  is 


Fig.  9. — A,   muscular  fibers  of  voluntary  muscle  ;  B,  one  fiber  broken  to 
show  its  investing-  sheath  ;  C,  cells  of  involuntai-y  muscle. 

invested  with  a  thin  membrane  of  connective  tissue  (Fig. 
9).  When  a  muscle  contracts  the  fibers  are  observed  to 
shorten  and  thicken  (Fig.  9,  A,  B,  C).     , 

Varieties. — There  are  two  kinds  of  muscle,  voluntary 
and  involuntary.  The  fibers  of  the  one  are  transversely 
striated  or  minutely  wrinkled;  the  fibers  of  the  other 
are  not  thus  wrinkled  (Fig.  9,  C). 

Of  all  tissues,  muscle  is  that  one  in  which  the  original 
cell  structure  is  most  obscured  by  modification  for  func- 
tion.    In  perfectly  formed  striated  muscle  there  is  no  ap- 


GENERAL    STRUCTURE    OF    ANIMALS. 


19 


/Wfl 


pearance  of  cell  structure  visible;  but  its  essential  cell 
structure  is  seen  in  the  embryonic  development  of  muscle. 
Fig.  10  shows  the  fibers  of 
the  muscle  of  the  embryonic 
lieart  of  a  monkey.  The  for- 
mation of  fibers  by  fusion  of 
nucleated  --cells  is  evident. 
W'e  shall  discuss  this  again  in 
connection  with  the  physiol- 
ogy of  muscle  as  an  organ. 

5.  Nerve  Tissue. — This 
is  the  highest  and  most  won- 
derful tissue  of  the  animal 
body  ;  not,  however,  so  much 
in  structure  (for  it  is  perhaps 
less  specialized  than  muscle), 
but  in  its  function,  ^^'ith  it, 
in  some  way  imperfectly  un- 
derstood, is  connected  the  Fig.  10.— Muscular  fibers  of  the 
J-    •  •  heart  of  the  embr\'o  monkev. 

transmission  of   impressions  After  Kent. 

from  the  external  world   to 

the  consciousness,  and  from  the  will  back  again  to  the 

e.Kternal    world.     With   it   also    is  connected   sensation, 

consciousness,  will,  thought,  and  all  the  higher  faculties 

of  the  mind. 

Varieties. — There  are,  again,  two  kinds  of  tissue  in 
nerve  tissue,  \\z.,  gray  g?anulcir  and  white  fibrous.  The 
gray  granular  consists  of  nucleated  cells  of  different 
forms  and  sizes,  apparently  connected  with  one  another 
by  interlacing  fibers  (Fig.  11).  The  white  fibrous  con- 
sists of  very  slender  parallel  fibers,  of  great  length,  con- 
nected with  gray-matter  cells.  The  characteristic  func- 
tion of  the  gray  granular  matter  is  the  origination  of 
nerve  force;  the  function  of  the  white  fibrous  matter 
is  the  transmission  of  the  same.     The  one  mav  be  com- 


20      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 


-M 


1 


pared  to  battery  cells  generating,  the  other  to  the  wires 
transmitting,  electric  energy.  The  one  is  found  only  in 
the  ncn'e  centers,  such  as  the  brain,  the  spinal  cord,  etc.  ; 

the  other  also  and  most  char- 
acteristically in  the  nerves 
proper.  We  will  discuss  this 
more  fully  in  connection  with 
the  physiology  of  the  nervous 
system. 

6.  Epithelial  Tissue. — 
The  whole  surface  of  the  ani- 
mal body,  both  external  and 
internal,  is  covered  with  a 
pavement  of  living  nucleated 
cells.  These  are  called  epithe- 
lial cells.  In  the  higher  ver- 
tebrates those  on  the  outer 
surface  dry  up,  become  more 
or  less  indurated,  and  are 
called  epidermal.  Those  on 
the  interior  surfaces  or  mu- 


I  \  *      Si 


W  ^ 


I 


\^\ 


"  f 


Fig.   II.— \eiticai  section  of  a     cous    membranes  always    re- 

convolution  of  the  brain,  show-      .     •        ..'      •  c-        „t'  „       a- 

ing  the  cells  of  the  gra.;  gran-     ^ani   tneir   soft,  active  condi- 
uiar  matter  (gg  )  giving  out     tion.      They    are    constantly 

fibers   which  go  to   form   the  .  .  . 

'white  fibrous  matter  (w/j  be-     dying,  dissolving  and  passing 

low.     After  Luys.  c  ^, 

away  as  mucus  of  the  mucous 
surfaces,  and  as  constantly  being  born  and  the  pavement 
renewed.     It  is  the  least  modified  of  all  the  tissues. 

Function. — Their  function  is  perhaps  the  most  impor- 
tant in  the  body,  viz.,  the  absorption  of  matter  (food) 
from  the  e.xternal  world  into  the  interior  of  the  body, 
and  the  elimination  of  waste  matter  from  the  body  into 
the  external  world.  In  other  words,  the  whole  exchange 
of  matter  between  the  exterior  and  interior  is  carried  on 
through  this  tissue. 


GENERAL    STRUCTURE    OF   ANIMALS. 


21 


Varieties. — This  pavement  of  living  cells  may  be  of 
different  patterns;  most  usually  the  cells  are  somewhat 
rounded  (cobble-stone  pavement).  Sometimes  they  are 
flat,  polygonal,  and  fitted  together,  like  a  tesselated  pave- 
ment.     Sometimes  they  are  elongated  and  set  on  end 


Fig.  12. — Different  forms  of  epithelial  cells  :  A,  rounded  ;  B,  flat  tesselated  ; 
C,  columnar  ;  D,  ciliated. 

(columnar  epithelium),  like  wooden  block  pavements. 
Sometimes  these  living  cells  are  provided  with  cilicne, 
which  are  in  continual  waving  motion  and  determine 
currents  on  the  surface  in  definite  directions  (ciliated 
epithelium)  (Fig.  12,  A,  B,  C,  D). 

For  convenience  of  reference  we  give  a  schedule  of 
the  principal  kinds  and  their  varieties  : 


22      I'HVSIOLOGN'    AND    MUKl'IKJLuGV    UF    ANIMALS. 

^  Areolar. 

1.  Connective ■]  Fibrous. 

'  Dermoid. 

(  Permanent. 

2.  Cartilaije -,  ,,, 

*  (  lemporary. 


Bone  proper. 
Bone -!  Dentine. 


(  Enamel 


(  Striated. 

4.   Muscle -^  X T       ,   .   ^    , 

I  Nonstriated. 

\  Gray  granular. 

^'   '  I  White  fibrous. 

r  Rounded. 

.  ,    ,.  Tesselated. 

6.    Epithelium <  „   , 

'  Columnar. 

I  Ciliated. 

Thus,  then,  there  are  composing  the  animal  body  six 
different  kinds  of  tissues  with  their  varieties,  each  with 
a  different  function,  and  all  co-operating  to  produce  one 
end,  viz.,  the  conservation  of  the  life  and  happiness  of 
the  organism.  This  is  the  type  and  expression  of  or- 
ganization, but  it  is  realized  only  in  the  higher  animals. 
As  we  go  down  the  scale  either  in  the  animal  series  or  in 
the  embryonic  series,  one  tissue  after  another  disap- 
pears, first  bone,  then  cartilage,  then  muscle,  then  nerve, 
until  only  unmodified  cell  aggregate  remains,  and  still 
lower  only  a  single  unmodified  cell  remains.  The  cor- 
responding functions  merge  into  one  another,  and  at 
the  same  time  become  less  and  less  perfect,  until  every 
part  performs,  but  very  imperfectly,  a//  the  functions  of 
life.  Or,  taking  the  reverse  order,  which  is  the  order  of 
evolution  :  first  there  is  only  one  cell  performing  all  the 
necessary  functions  of  life,  but  very  imperfectly;  next 
an  aggregate  of  unmodified  and  therefore  similar  cells 


GENERAL    STRUCTURE    OF    ANIMALS.  23 

all  performing  similar,  i.  e.,  all  the  functions,  but  imper- 
fectly. Then  the  process  of  differentiation  commences 
and  proceeds.  Some  cells  take  on  a  special  form 
adapted  to  the  performance  of  a  special  function,  say 
contraction,  and  aggregate  to  form  a  tissue,  muscle. 
Other  cells  take  other  forms  and  aggregate  to  form 
other  tissues  adapted  to  perform  other  characteristic 
functions,  until  finally  in  the  mature  condition  of  the 
highest  animals  each  kind  of  cell  performs  but  one  func- 
tion, but  performs  it  very  perfectly.  Thus  a  muscular 
fiber  can  do  nothing  else  but  contract.  A  nerve  cell 
gives  no  other  sign  of  life  but  feeling,  etc.  This  whole 
process  of  modification  of  form  and  limitation  and  per- 
fecting of  function,  or  division  of  labor,  is  called  the 
law  of 'differentiation.  It  is  the  most  fundamental  and 
universal  law  of  evolution. 

Observe  here — and  the  same  is  true  of  all  differentia- 
tions— two  ideas  which  must  be  kept  distinct  in  the  mind, 
viz.,  (i)  identity  of  plan  or  community  of  origin — in  this 
case  cellular  structure — and  (2)  adaptiir  modification  for 
various  functions. 

So  much  for  tissues.  But  physiology  is  concerned 
with  functions,  and  functions  are  usually  and  properly 
treated  in  connection  with  organs,  such  as  muscle,  brain, 
gland,  etc.  The  body  consists  primarily  of  organs. 
Thus  cells  aggregate  to  form  tissues,  tissues  aggregate 
to  form  organs,  and  organs  aggregated  form  the  animal 
body. 

SECTION    IV. 

ORGANS    .AND    FUXCnOXS    OF    THE    ANIM.AL    Bcmv. 

Classification  of  Functions. — The  functions  of 
the  animal  body  are  of  two  general  kinds,  viz.,  functions 
of  animal  life  and  functions  of  vegetative  or  organic  life. 


24     PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

or,  more  briefl}-,  animal  functions  and  organic  functions. 
The  animal  functions  are  those  which  are  distinctive  of 
animals.  The  organic  functions  are  those  which  are 
possessed  in  common  by  animals  and  plants,  and  are 
therefore  coextensive  with  life.  Thus  an  animal  may 
be  regarded  as  a  plant  with  certain  higher  and  distinc- 
tive functions  superadded.  So  also  man  may  be  re- 
garded as  an  animal  with  certain  higher  and  distinctive 
functions  or  faculties  superadded.  As  the  idea  of  ani- 
mal life  is  realized  in  proportion  as  the  distinctive  ani- 
mal functions  predominate  over  the  organic,  so  also 
the  idea  of  human  life  is  realized  only  in  proportion  as 
the  distinctive  human  faculties  predominate  and  control 
the  animal. 

Now  these  superadded  distinctive  animal  functions 
ure  all  concerned  with  conscious  action  and  reaction  be- 
tween the  external  world  and  the  organism.  They  are 
therefore  divisible  into  sensation  and  consciousness 
(action)  on  the  one  hand,  and  will  and  voluntary  mo- 
tion (reaction)  on  the  other.  These  are,  however,  very 
closely  related,  being  both  connected  with  the  nervous 
system.  The  organic  functions — viz.,  those  common  to 
all  life — are  subdivided  into  two  more  widely  separated 
groups,  viz.,  the  nutritive  and  reproductive.  The  nutritive 
functions  are  all  that  assemblage  of  functions  which 
co-operate  for  the  conservation  of  the  life  of  the  indi- 
vidual;  the  reproductive.,  all  that  assemblage  of  functions 
which  co-operate  for  the  conservation  of  the  kind,  even 
though   the  individual  must  die. 

.    .       ,  r  Sensation  and  consciousness. 

(  Animal. ...  J 

Functions     )  (  ^^^'^  ''^"^  voluntary  motion. 

i  r\         ■  (  Nutritive  functions. 

'  Organic.  . .  j 

I  Reproductive  functions. 

Order  of  Treatment. — Of   these  groups  we  shall  take 

up  animal  functions  first,  because  in  the  higher  animals 


GENERAL   STRUCTURE   OF   ANIMALS.  25 

these  functions  dominate  the  others.  These  will  form 
the  subject  of  Part  I.  In  Part  II  we  shall  treat  the 
first  subdivision  of  the  organic  functions,  viz.,  the  nutri- 
tive functions,  or  those  which  have  to  do  with  the  con- 
servation of  individual  life.  There  ought  to  be  a  Part 
III,  treating  of  all  that  assemblage  of  organs  and  func- 
tions concerned  with  the  conservation  of  the  race  or 
species;  but  this  is  so  vast  a  subject  that  it  would  re- 
quire a  separate  treatise. 


PART    I. 

ORGANS  AND  FUNCTIONS  OF  ANIMAL    LIFE. 

These  must  be  treated  under  four  groups  or  systems 
of  organs,  viz. :  i.  Nervous  system.  2.  Sense  organs. 
3.  Muscular  system.  4-  Skeletal  system.  These  are 
closely  connected  in  the  performance  of  the  functions  of 
animal  life.  The  general  way  in  which  they  co-operate 
is  shown  in  the  diagram  (Fig.  13). 


Fig.  13. — Diagram  showing  essential  parts  of  an  apparatus  of  exchange  be- 
tween the  external  world  and  consciousness  :  NC,  nerve  center  ;  J'c,  sen- 
sory cell  ;  s/\  sensory  fiber  ;  SS,  sensory  surface  ;  inc,  motor  cell ;  tn/, 
motor  fiber  ;  M,  muscle.     Arrowheads  show  the  direction  of  transmis- 


We  have  (i)  an  impression  on  a  sense  organ,  SS\  (2) 
a  transmission  imvard  along  a  sensory  fiber  to  a  nerve 
center  N.C.,  say  the  brain  ;  (3)  a  change  of  some  kind 
in  a  sensory  cell,  s.c,  which  awakens  conscious  sensation  ; 
(4)  an  influence  of  some  kind  transferred  by  a  connect- 
ing fiber  to  a  motor  cell,  m.c. ;  (5)  an  impulse  transmitted 
outward  along  a  motor  fiber,  w./.,  to  a  muscle,  J/,  and  de- 
26 


ORGANS   AND    FUNCTIONS   OF   ANIMAL    LIFE.       2/ 

termining  (6)  muscular  cuntraction  and  motion  and 
changes  in  the  external  world.  The  skeleton  acts  as 
levers  to  make  the  motion  more  rapid,  precise,  and  ef- 
fective. Thus  the  sense  organs  may  be  regarded  as  re- 
ceptive organs  of  sensation  and  consciousness,  and  the  mus- 
cles and  skeleton  as  executive  organs  of  the  7vill,  and 
the  whole  as  an  instrument  of  action  and  reaction  be- 
tween the  external  and  the  internal  world. 

Therefore  the  necessary  parts  of  an  instrument  of 
communication  between  the  outer  and  the  inner  world 
are  (i)  two  kinds  of  cells  in  the  nerve  center,  viz.,  a 
sensory  cell  and  a  motor  cell  with  connecting  fiber  be- 
tween ;  (2)  two  kinds  of  transmitting  fibers,  the  one 
sensory,  transmitting  inward,  the  other  motor,  trans- 
mitting outward ;  and  (3)  two  kinds  of  nerve-fiber  end- 
ings, one  in  a  sensitive  surface  or  a  sense  organ,  the 
other  in  a  contractile  tissue  or  muscle.  Each  cell,  sen- 
sory or  motor,  with  its  fiber  and  its  ending  is  called  a 
neurone  or  neurocyte.  The  connection  between  a  sensory 
and  a  motor  neurone,  until  recently,  was  supposed  to  be 
continuous  and  permanent,  as  represented  in  the  figure  ; 
but  now  it  is  believed  to  be  by  contact  of  branching  pro- 
cesses, and  perhaps  only  during  stimulation.  This  will 
be  explained  more  fully  hereafter. 

Of  the  four  systems  mentioned  as  concerned  in  ani- 
mal functions,  viz.,  nervous  system,  sense  organs,  mus- 
cles, and  skeleton,  the  fundamental  one  is  the  nervous. 
The  others  may  be  regarded  as  appendages  of  this  one. 
We  therefore  take  this  first. 

Order  of  Treatment. — There  are  two  modes  or 
orders  of  taking  up  the  subject  of  comparative  physiol- 
ogy and  morphology.  We  may  begin  with  the  lowest 
and  go  up  the  scale  ;  this  is  the  order  of  evolution. 
Or  we  mav  begin  with  man  and  pass  down  the  scale. 
If    our    subject    were    mainlv    morphology    the    former 


28      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

might  possibly  be  the  best,  although  even  then,  it  is  per- 
haps doubtful;  but  in  physiology  there  can  be  no  doubt 
that  the  latter  is  best.  It  is  so  for  two  reasons:  first, 
because  we  take  hold  at  once  of  the  interest  of  the 
student,  and,  second,  because  functions  are  fully  sepa- 
rated and  declared  and  therefore  intelligible  only  in  the 
higher  animals.  As  we  go  down  the  scale  functions  are 
more  and  more  merged  into  one  another,  and  therefore 
more  and  more  indistinct,  until  in  the  lowest  animals 
each  part  performs  all  the  functions,  but  \n  so  imperfect 
a  way  that  it  is  impossible  to  understand  them  unless 
we  have  already  studied  them  in  their  separated  and 
perfect  form  in  the  higher  animals.  "  In  higher  animals 
the  functions  rise  to  the  surface;  in  lower  animals  they 
are  deeply  buried.  We  grope  in  vain  unless  we  find  the 
key  by  the  study  of  the  higher  animals."  * 

Our  plan,  then,  will  be  to  take  up  each  system  of 
organs  first  in  man;  then,  running  down  the  scale  of 
vertebrates^  show  the  modifications  and  simplifications 
which  we  find  there.  Then  we  shall  take  the  other  de- 
partments of  the  animal  kingdom  and  treat  them  in  the 
same  way,  but  much  more  cursorily. 

*  Foster,  Nature,  vol.  Ivi,  p.  437,  1897. 


CHAPTER    I. 


THE    NERVOUS    SYSTEM    OF    MAN. 


The  nervous-  system  of  man,  and  indeed  of  all  ver- 
tebrates, may  be  divided  into  two  subsystems,  viz.,  the 
cerebrospinal  dir\6.  ihe  ganglionic.  Their  relations  to  one 
another  are  shown  in  a  subsequent  figure  (Fig.  38, 
page  68).  We  put  aside  for  the  present  the  ganglionic 
system. 

THE    CEREBRO-SPIN.AL    SYSTEM. 

General  Plan. — The  general  plan  of  structure  of 
this  system  in  man,  and  indeed  in  all 
vertebrates,  is  simply  expressed  as  a 
continuous  tract  or  axis  of  gray  matter 
extending  nearly  the  whole  length  of 
the  body,  from  which  run  off  in  pairs 
bundles  of  fibers  (nerves)  going  to  every 
part  of  the  body,  as  shown  in  the  dia- 
gram (Fig.  14).  In  the  lower  verte- 
brates there  is  very  little  more  than  this, 
but  in  the  higher  vertebrates,  and  espe- 
cially in  man,  this  simple  plan  is  ob- 
scured by  the  enormous  development  of 
the  anterior  part  as  a  brain,  as  shown 
in  the  dotted  outline.  This  continuous 
tract  is  called  the  cerebro-spinal  axis. 

The  cerebro-spinal  axis  may  be 
again  subdivided  into  the  brain  and  the 
spinal  cord \   so   that  the  subject  of  the 


Fig.  14. — Diagjam 
showing  the  pen- 
era!  plan  of  struc- 
ture of  the  verte- 
brate nervous  syS' 
tem. 


29 


30 


I'lIVSlOLOGV    AM)    .MORl'lIOLOGV    OF    ANIMALS. 


cerebro-spinal  system  may  be  treated  under  three  heads 
of  (i)  the  brain^  (2)  the  spinal  cord,  and  (3)  the  nerves. 
The  brain  and  spinal  cord  are  centers,  the  nerves  are 
conductors.  The  first  two  contain  gray  matter  as  well 
as  white,  the  last  white  matter  alone.  The  first  two  are 
generators  of  nerve  energy,  the  fibers  of  the  third  are 
transmitters  only.  The  former  may  be  likened  to  bat- 
tery cells,  the  latter  to  conducting  wires. 

SECTION    I. 
Brain  of  Afati. 

We  can  give  only  such  general  description  as  is  neces- 
sary to  explain  physiology. 

Skull. — The  brain  is  inclosed  in  a  bony  bo.x  consist- 
ing of  many  pieces  fitted  together  by  sutures  with  inter- 
locking teeth.  The  growth  of  the  skull  to  accommo- 
date the  growing  brain  takes  place  along  these  sutures. 
The  sutures  finally  consolidate  and  the  brain  can  grow 
no  more.  The  age  of  consolidation  is  later  in  the 
higher  races,  and  is  probably  also  later  in  educated 
men.* 

Membranes. — Take  off  the  skull,  and  beneath  we 
see  the  brain  still  enveloped  by  its  membranes.  These 
are  (i)  the  dura  mater,  a  thick,  strong,  fibrous  membrane. 
This  invests  the  brain  and  dips  in  and  separates  all  the 
great  divisions  of  the  brain,  but  not  the  convolutions. 
It  carries  also  the  large  blood  vessels  of  the  brain. 
Beneath  and  more  closely  investing  the  brain,  passing 
between  not  only  the  larger  but  also  the  smaller  di- 
visions, and  even  dipping  down  between  the  convolu- 
tions of  the  surface  and  carrying  the  smaller  blood  ves- 
sels which  penetrate  the  substance  of  the  brain  itself,  there 

^  Gahon,  Nature,  vol,  xxxviii,  p.  14,  1888, 


THE    NERVOUS   SYSTEM    OF    MAN. 


31 


is  seen  (2)  a  more  delicate  membrane  called  the />ta  mater. 
Between  these  and  uniting  them  is  still  a  third  mem- 
brane (3)  the  arachnoid.  It  is  intlammation  of  these 
membranes  rather  than  of  the  brain  itself  that  consti- 
tutes the  more  acute  forms  of  brain  disease  attended 
with  violent  delirium. 

Main  Parts  of  the  Brain,     i.  Cerebrum. — Take 
out  the  brain   from  the  skull   and  place  it  on  the  table 


Fig.    15. — Cerebrum  seen  from  above. 

and  remove  the  membranes.*  Looking  down  on  it  from 
above  we  see  nothing  but  a  great  hemispherical  irregu- 
larly convoluted  mass — the  cerebrum — divided  along 
the  middle  into  two  equal  halves.     These  are  the  right 


*  In  the  absence  of  brain  an  Auzoux  model  will  serve  an  excel- 
lent purpose. 
4 


32 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


and  left  cerebral  hemispheres  (Fig.  15).  The  trench  that 
divides  them  is  about  two  inches  deep  and  occupied  by 
an  extension  of  the  membranes.  The  two  hemispheres 
are  connected  below  by  a  band  about  half  an  inch  thick 
— the  corpus  callosum.  The  cerebrum  constitutes  about 
four  fifths  of  the  whole  brain. 

2.  Cerebellum. — Lifting  the  cerebrum  from  behind, 
the  next  most  important  part  brought  into  view  imme- 
diately beneath  the 
hinder  part  of  the 
cerebrum  is  the  cere- 
bellum. This  is  also 
a  double  organ  like 

f    ■    /i    W^      ^^%^  f  f /"^yf     the  cerebrum, but  the 

t^  \,^u^ ^^^^  ^^^JMk^^l^     two    halves   are   not 

so  deeply  divided  by 
the  membranes.  The 
peculiar  leafiike  ar- 
rangement of  the 
convolutions  will  be 
observed.  A  side 
view  of  the  brain 
(Fig.  16)  shows  the 
cerebellum  beneath  the  under  part  of  the  cerebrum. 

3.  Medulla  and  Pons. — Lying  beneath  the  cere- 
bellum, as  if  this  latter  had  grown  out  of  it,  is  an  en- 
larged continuation  of  the  spinal  cord  within  the  skull. 
This  is  called  the  medulla.  Beneath  this  again,  with 
fibers  running  across  the  medulla  and  connecting  the 
two  sides  of  the  cerebellum,  is  l\\& pons  Varolii  (bridge  of 
Varolius).  This  can  only  be  seen,  however,  by  turning 
the  brain  over  so  as  to  see  the  under  side  (Fig.  17). 

4.  Optic  Lobes. — Lifting  the  cerebrum  from  behind 
still  higher  and  looking  farther  forward,  the  optic  lobes 
are  bronirht  into  view  in  front  of   the  cerebellum.     In 


Fig.  16. — Side  view  of  the  brain  :  cr,  cere- 
brum ;  cb^  cerebellum  ;  ;«,  medulla  ;  s,  fis- 
sure of  Sylvius  ;  r,  fissure  of  Rolando. 


THE    NERVOUS   SYSTEM    OF    MAN. 


33 


human  anatomy  these  are  called  the  corpora  quadri- 
gemina,  because  they  consist  of  two  pairs  of  rounded  emi- 
nences, but  in  comparative  anatomy  they  are  called  the 


__i> 


Fig.  17. — View  of  brain  from  below  :  cr,  cerebrum  :  cb,  cerebellum  ;  m, 
medulla ;  /,  pons,  showing  the  origins  of  the  nerves ;  ol,  olfactory,  and 
f/,  the  optic  nerves. 


optic  lobes,  because  connected  with  the  sense  of  sight. 
They  are  small  and  inconspicuous  in  the  human  brain, 
but  in  the  lower  vertebrates  they  may  be  larger  even 
than  the  cerebrum. 

5.  Thalamus. — Lifting  the  hinder  part  of  the  cere- 
brum still  higher  and  looking  as  far  forward  as  possible, 
we  see  two  pairs  of  much  larger  rounded  masses.  These 
are  the  thalamus  (the  first  pair)  and  the  corpus  striatum 
(the  second  pair).  We  shall  often  speak  of  these  to- 
gether as  the  thalamus  (Fig.  18). 

It  would  appear,  then,  but  will  become  far  more  evi- 
dent   presently,   that  the    spinal  cord    enters  the    skull 


34 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


and  runs  along  its  base,  forming  successive  swellings 
and  outgrowths  in  its  course  forward.  First  the  me- 
dulla with  its  outgrowth,  the  cerebellum,  then  the  optic 

lobes  with  their  four  swell- 
ings atop,  then  the  thala- 
mus with  its  four  swellings, 
and  lastly  the  cerebrum; 
but  this  last  has  grown 
backward  and  covered  all 
the  other  parts  and  thus 
obscured  the  real  structure 
of  the  brain.  This  will  come 
out  more  clearly  presently. 
Convolutions  of  the 
Brain. — The  surface  of  the 
cerebrum  is  diversified  with 
irregular  folds  (convolu- 
tions) with  deep  trenches 
between.  Into  the  trenches 
enter  the  pia  mater,  but  not 
the  dura  mater.  There  are 
a  few  larger  trenches  called 
fissures  dividing  the  cere- 
bral hemispheres  into  lobes. 
The  most  conspicuous  of  these  is  the  fissure  of  Sylvius 
(Fig.  i6,  s).  It  commences  well  forward,  runs  backward 
and  upward,  separating  the  posterior  lobe  from  the  rest 
of  the  cerebrum.  Another  is  the  fissure  of  Rolando  (Fig. 
i6,  r),  which  separates  the  rest  of  the  cerebrum  into  two 
parts.  By  these  two  fissures  the  cerebrum  is  divided 
into  three  lobes — anterior,  middle,  and  posterior,  or 
frontal,  parietal,  and  occipital. 

The  cerebellum  is  convoluted  also,  but  in  a  more 
regular  way,  being  deeply  separated  into  parallel  lami- 
nae or  leaves.     All  the  other  parts  are  smooth. 


Fig.  i8. — A  cross-section  of  the  lifted 
cerebrum  :  Longitudinal  section 
of  the  cerebellum,  showing  optic 
lobes,  ol  \  thalamus  and  corpora 
striata,  th.     After  Dalton. 


THE    NERVOUS    SYSTEM    OF    .MAN. 


35 


Interior  Structure. — So  much  may  be  seen  without 
cutting,  but  on  making  section  we  find  at  once  in  all 
parts  the  two  kinds  of  nerve  matter  already  spoken  of 
(page  19),  viz.,  the  gray  granular  and  the  white  fibrous; 
but  the  relative  positions  of  these  two  kinds  are  different 
in  the  different  parts.  In  the  two  largest  parts,  viz.,  the 
cerebrum  and  cerebellum,  the  gray  matter  is  on  the  out- 
side and  the  white  fibrous  matter  on  the  inside.  In  these 
the  surface  gray  matter  follows  all  the  inequalities  of 
the  surface  convolutions — in  fact,  it  is  evident  that  the 
convolutions  are  a  device  to  increase  the  quantity  of 
gray  matter.     In  the  case  of  the  cerebellum  the  com- 


FiG.  ig.- — Section  of  cerebellum  showing;  arbor  vitae. 

plexity  of  the  infoldings  of  the  surface  gray  matter  is  so 
great  as  to  give  rise  on  section  to  the  peculiar  appear- 
ance called  arbor  vitcc  (Fig.  19). 

In  all  the  other  parts  mentioned,  viz.,  the  medulla, 
the  optic  lobes,  and  the  thalamus,  the  gray  matter  is  in 
the  center  and  the  white  matter  on  the  outside. 

Microscopic  Structure. — As  already  explained 
(pages  19  and  27),  the  gray  matter  consists  of  cells  of 
various  sizes  and  shapes,  giving  out  fibers,  some  connect- 
ing with  other  cells,  and  some  going  to  form  the  white 


36 


PHYSIOLUGY    AND    MORPHOLOGY    OF    ANIMALS 


fibrous  matter  (Fig.  20).  The  white  fibrous  matter  seems 
to  be  made  up  wholly  of  slender  fibers  which  come  from 
the  cells  of  the  gray  matter.     We  may   imagine  these 

fibers  coming  from  the  sur- 
face gray-matter  cells,  con- 
verging to  form  the  white 
matter  of  the  brain,  and  then 
passing  out  of  the  skull  to 
form  the  spinal  cord,  to  be 
distributed  everywhere.  Or, 
conversely,  we  may  conceive 
fibers  of  the  spinal  cord  com- 
ing into  the  brain  and  diverg- 
ing to  end  in  the  cells  of  the 
surface  gray  matter.  Then, 
last  of  all,  these  gray-matter 
cells  send  out  each  of  them 
fibers  which  connect  with 
those  of  other  gray-matter 
cells.  Now  it  is  probable  that 
such  a  cell  with  all  its  fibers, 
both  those  connecting  with 
other  cells  and  the  long  fiber 
(axis  cylinder)  connecting 
with  other  parts  of  the  body, 
together  constitute  one  indi- 
vidual cell.  Such  an  individual  element  of  nerve  matter 
is  called  a  neurone.  On  this  view  the  brain,  and  indeed 
the  whole  nerve  system,  may  be  regarded  as  naught  else 
than  a  collection  of  neurones  intricately  connected.  The 
fibers  connecting  neurones  are  not  simple,  but  branching 
(dendrites),  and  the  connection  is  not  continuous,  but 
by  contact.  They  do  not  unite,  but  only  touch  fingers 
or  interlace  dendrites  (Fig.  21).* 

*  Professor  Turner,  British  Association  Address,  1897.     Mathi- 
as-Duval,  Rev.  Sci.,  voL  ix,  p.  321,  1898. 


Fig.  20. — \  ertical  section  of  a 
convolution  of  the  brain,  show- 
ing the  cells  of  the  gray  gran- 
ular matter  (g g  )  giving  out 
fibers  which  go  to  form  the 
white  fibrous  matter  {w  /  )  be- 
low.    After  Luys. 


THE    NERVOUS   SYSTEM    OF    MAN. 


37 


Embryonic  Development  of  Brain. — The  funda- 
mental fact  that  the  brain  may  be  regarded  as  an  inter- 
cranial  continuation  of  the 
spinal  cord,  with  swellings 
and  outgrowths  atop,  is 
made  evident  by  its  em- 
bryonic development.  The 
following  figures  give  the 
stages  of  this  develop- 
ment. In  the  very  early 
stages  the  brain  is  a  direct 

continuation  of    the  spinal     Fig.  21.— Diagram  showing  the  inter- 
lacing of  dendrites  of  neurones. 

cord  and  consists  of  three 

hollow  swellings  or  vesicles.     These  are  what  afterward 

become  medulla  (i),  optic  lobes  (2),  and  thalamus  (3). 


^'^ 


We  shall  call  these  the  hinJbrain,  the  midbrain,  and  the 
forebrain  (Fig.  22).  The  ne.xt  step  is  the  outgrowth  of 
the  cerebrum  [cr)  and  olfactory  lobes  {of)  from  the  fore- 


FiG.  23. 


3« 


J'in  SIOLOCV    AND    MORPHULUUV    Of    ANIMALS 


brain  (No.  3),  the  cerebellum   (^Z')   from   the  hindbrain 
(No.  i),  and  from  the  midbrain  (No.  2)  the  formation 


Fig.  24. 

or  outgrowth  of  the  swellings  characteristic  of  the  optic 
lobes  (Fig.  23).  The  next  step  is  that  the  outgrowths 
from  I  and  3 — i.  e.,  the  cerebellum  and  cerebrum — in- 
crease enormously.  This  is  especially  true  of  the  cere- 
brum, which,  commencing  as  the  foremost  in  the  series, 
grows  forward,  sidewise,  and  especially  backward,  cov- 
ering first  the  thalamus  (Fig.  24),  then  the  optic  lobes 
(Fig.  55),  and  finally  the  cerebellum,  and  thus  masks  the 


true  structure   of    the   brain    (Fig.    26).     The   following 
schedule  gives  the  parts  as  brought  out  by  embryology. 


THE    MEKVOUS    SYSTEM    OF    MAN.  39 

The  italicized  are  the  basic  parts,  the  others  being  out- 
growths. 

f  Olfactory  lobes. 

Forebrain    -|  Cerebrum. 

'  Thala7nus. 
Midbrain Optic  lobes. 

(  Medulla. 
Hindbrain j.  Cerebellum. 

(  Pons. 

See  also  the  strange  upward  and  downward  growths 
from  the  thalamus.  These  are  the  pituitary  (//)  and  the 
pineal  (/>//)  glands.     We  shall  speak  of  these  again. 


Fig.  26. 


The  Distinctive  Functions  of  these  Parts. — We 

determine  the  functions  of  these  several  parts  partly  by 
observation  of  injury  or  disease  affecting  them  in  case 
of  man,  but  mainly  by  removal  of  them  in  case  of  the 
lower  animals. 

Cerebrum. — For  example,  if  the  cerebrum  be  removed 
from  the  brain  of  pigeons,  the  bird  continues  to  live,  but 


40 


PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


remains  in  deep,  comatose  sleep,  unconscious  and  inca- 
pable of  initiating  any  movement  whatsoever.  It  stands; 
if  pushed,  it  will  recover  itself;  if  thrown  into  the  air, 
will  fly  a  little  way  and  alight,  but  lapses  again  into 
coma.  If  food  be  put  into  the  mouth,  it  swallows  and 
digests  it,  but  does  not  voluntarily  take  it.  Indeed,  it 
will  starve  in  the  presence  of  abundant  food.  From 
these  experiments  it  is  believed  that  the  cerebrum  is  the 
seat  of  consciousness  and  conscious  sensation,  of  voli- 
tion and  voluntary  motion,  and  a  fortiori  oi  all  the  still 
higher  functions,  such  as  intelligence,  etc. 

Cerebellum. — If  the  cerebellum  alone  be  removed,  the 
animal  seems  to  be  perfectly  conscious,  and  tries  to  make 
its  usual  movements  of  standing,  walking,  flying,  etc., 
but  can  not  do  so  successfully.  It  can  not  stand  or 
walk  steadily.  It  flutters,  but  can  not  fly.  All  its  move- 
ments are  voluntary,  but  uncertain  and  staggering.  It 
is  concluded,  therefore,  that  the  function  of  the  cerebel- 
lum is  the  co-ordination  of  muscular  contraction.  "  It  is 
the  reflex  organ  of  equilibration."*  In  the  acts  of 
standing,  walking,  flying,  etc.,  very  many  muscles  are 
used.  The  contraction  of  these  is  initiated  by  the  will, 
whose  seat  is  in  the  cerebrum,  but  they  must  be  per- 
fectly co-ordinated  in  order  to  accomplish  any  complex 
act  successfully.  This  is  done  by  the  cerebellum.  The 
staggering  of  drunkenness  is  the  partial  paralysis  of  the 
cerebellum. 

Medulla.— 'Y\\\'~,  is  the  connecting  link  between  the 
other  parts  of  the  brain  and  the  spinal  cord,  and  through 
the  cord  with  the  rest  of  the  body.  For  this  reason,  then, 
its  importance  is  supreme.  Again,  the  nerves  control- 
ling the  most  vital  functions  of  the  body,  such  as  those 
of  the  lungs  and  the  heart,  originate  in  the  gray  matter 

*  Rev.  Sci.,  vol.  viii,  p.  503,  1897. 


THE    NERVOUS   SYSTEM    OI-    MAN. 


41 


of  this  part.  It  is  therefore  the  part  most  immediately 
necessary  to  Ufe.  I'he  gray  matter  of  the  medulla  is 
the  center  controlling  autoviatically  the  most  vital  pro- 
cesses of  the  body.  Removal  of  this  produces  immedi- 
ate death. 

Optic  Lobe. — This  is  probably  the  immediate  con- 
troller of  the  sense  of  sight;  its  destruction,  therefore, 
destroys  that  sense.  The  optic  nerve,  coming  from  the 
eye,  sends  one  root  to  the  optic  lobes  and  another  to  the 
thalamus.  The  latter  sends  an  influence  to  the  visual  area 
of  the  cerebrum.     We  will  exj^lain  this  more  fully  later. 

Thdlamus  and  Corpus  Striatum. — These  ganglia  are 
undoubtedly  very  important  and  very  necessary  to  life. 
Their  function  is  still  ob- 
scure, but  from  their  con- 
nection with  the  cerebrum 
on  the  one  hand,  and  the 
rest  of  the  body  on  the 
other,  they  seem  to  be  an 
intermediary  between  these 
two.  Sense  impressions 
from  surfaces  and  sense 
organs,  on  their  way  to  the 
cerebrum,  seem  to  pass 
through  the  thalamus  and 
receive  impulse  from  that 
organ ;  and  nnpulses  or 
mandates  from  the  cere- 
brum on  their  way  outward 
to  the  muscles  seem  to  pass 
through  the  corpus  stria- 
tum and  receive  fresh  impulse  there.  They  are  relay 
batteries  in  the  course  of  communication  between  brain 
and  body  (Fig.  27).  They  are,  moreover  and  especially, 
centers  of  semiautomatic  or  habitual  nun'ements.     There  are 


Fig.  27. — Dia.PTam  showing  supposed 
function  of  thalamus  and  corpus 
striatum  in  relation  to  the  cere- 
brum :  cs,  cerebral  senior}- ;  cm, 
motor  ;  csm,  corpus  striatum  ;  tJis, 
sensory  thalamus. 


4^ 


rilVSIOLOGV    ANT)    MORPHOLOGY    OF    ANIMALS. 


three  kinds  of  movements  in  the  animal  bod}^  viz.,  vol- 
untary, semivoluntary,  and  reflex.  The  cerebrum  pre- 
sides over  the  first — viz.,  the  distinctly  and  consciously 

voluntary  actions — such  as 
movements  undertaken  for 
the  first  time  and  requiring 
full  attention  and  distinct  ef- 
fort. On  the  other  hand,  the 
medulla  and  spinal  cord  pre- 
side over  the  purely  auto- 
matic or  reflex  movements 
— movements  wholly  with- 
drawn from  consciousness 
and  will,  like  those  of  respi- 
ration and  of  the  heart.     The 

Fig.  28. — Illustratinsj  function  of     ,1      1  .„       4.  -a 

thalamus:  ..sensory  fiber ;   .«,      thalamus     SCems     tO     preside 

motor  fiber; ///..sensory  cell  of    Qver      intermediate      move- 

thalarnus ;    csm^  motor   cell  of 

corpus  striatum ;  crs  and  crm,     ments — i.e..  Semiautomatic  or 

sensory  and  motor  cells  of  cere-      ■<      -u- .        in       ^u  c     ..         1 

jjyyjjj  ■'  habitual,  like  those  of  stand- 

ing, walking,  flying,  writing, 
speaking,  playing  on  a  musical  instrument,  etc.  All 
these  are  acquired  with  some  difficulty,  the  cerebrum 
presiding,  but  gradually  become  easier  and  easier  until 
they  require  only  the  most  general  superintendence  of 
consciousness  and  will.  If  anything  goes  wrong,  the 
cerebrum  takes  control  for  a  while  and  sets  things  right, 
and  again  the  movements  lapse  into  semiautomatism. 
It  is  as  if  the  cerebrum  gradually  taught  these  under- 
agents  or  employees — the  thalamus  and  corpus  striatum 
— to  do  the  work  themselves,  but  under  general  super- 
vision. To  compare  to  an  electric  apparatus,  it  is  as  if 
the  sense  impulse  goes  up  to  the  cerebrum  through  a 
sensory  cell  of  the  thalamus  and  comes  back  from  the 
cerebrum  through  a  motor  cell  of  the  corpus  to  the 
muscle,  but  a  part  of  the  current  sJiort  circuits  from  the 


THE    NERVOUS   SYSTEM    OF    MAN.  43 

thalamus  to  the  corpus  and  downward  to  the  muscle. 
This  short  circuiting  becomes  more  and  more  perfect 
until  only  a  little  overflow  goes  to  the  cerebrum,  and 
thus  keeps  it  aware  of  what  is  going  on.  This  view  is 
illustrated  by  the  diagram  (Fig.  28),  in  which  s  and  m  = 
sensory  and  motor  libers,  i/is  and  cs/?i  =  sensory  and 
motor  cells  of  thalamus  and  corpus  striatum,  and  crs  and 
crm  =  similar  cells  in  the  cerebrum.  The  arrows  show 
the  direction  of  the  nerve  current. 

Localization  of  Cerebral  Functions. — The  cerebrum  is 
the  highest  ganglion  of  the  brain,  and  therefore  we 
ought  to  expect  there  the  greatest  degree  of  differentia- 
tion and  localization  of  functions.  The  old  phrenology 
attempted  to  localiz-e  the  faculties  of  the  mind  ;  but  re- 
cently there  has  arisen  a  new  and  more  scientific  though 
far  less  ambitious  attempt  to  localize  not  indeed  the 
faculties  of  the  i/iinJ,  but  the  functiofts  of  the  cerebrum — 
i.  e.,  the  areas  of  the  cerebrum  receiving  and  appreci- 
ating sense  impressions  from,  and  the  areas  determining 
and  controlling  the  motions  of,  various  parts  of  the 
body.  The  conclusions  arrived  at  in  these  investigations 
are  based  almost  wholly  on  experiments  on  the  lower 
animals,  especially  the  monkey,  although  some  of  them 
have  been  confirmed  by  observations  on  man  in  cases  of 
injury  or  diseases  of  the  brain. 

These  investigations  are  as  yet  very  imperfect,  but 
some  reliable  results  have  been  attained.  Fig.  29  gives 
the  best  established  areas.  It  must  be  remembered  that 
many  of  our  movements  are  automatic  or  semiauto- 
matic. These  are  presided  over  by  the  lower  ganglia, 
such  as  the  thalamus,  the  optic  lobes,  or  the  medulla. 
Take  the  sense  of  sight,  for  example.  Many  of  our 
sight  impressions  do  not  rise  into  distinct  conscious- 
ness, and  yet  appropriate  actions  may  take  place. 
These   are   probably  determined    by   the   thalamus   and 


44 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


optic  lobes,  into  which  the  optic  roots  are  seen  to  enter. 
But  in  many  cases  we  consciously  observe  and  remem- 
ber the  impressions  of  sight — form  mental  images  of 
objects  seen.  In  such  cases  the  impression  is  sent  on 
from  the  thalamus  to  the  cerebrum.  The  area  to  which 
these  impressions  are  sent — visual  area — is  situated  in 


yuph. 


Fig.  29.- — Functional  areas  of  the  cerebrum  :  m,  motor  of  the  body  ;  j,  sen- 
sory of  the  body  ;  v^  visual ;  olf,  olfactory  areas  ;  apli^  motor  aphasia 
(speech  ;  a.aph^  auditory  aphasia;  v.apli,  visual  aphasia  (reading;; 
g.aph,  graphic  aphasia  (writing). 

the  posterior  lobes  and  marked  v.  Similarly,  auditory 
areas  are  marked  a.a,  general  sensation  areas  by  s,  gen- 
eral motor  areas  by  w,  and  olfactory  areas  by  olf. 

One  of  the  most  curious  and  interesting  of  these 
discoveries  is  that  of  the  speech  area,  aph.  This,  of 
course,  was  discovered  by  observation  on  man — not 
experiments  on  animals.  It  has  been  long  observed 
that  there  are  cases  in  which  a  patient  is  perfectly  intel- 
ligent and  knows  what  he  wants  to  say,  but  can  not  say 
it.  Such  an  affection  is  called  aphasia.  In  such  cases  it 
is  invariably  found  by  post  mortem  that  there  is  a  lesion 
of  a  particular  convolution  of  the  frontal  lobe,  especially 
of  the  left  side. 


THE    NERVOUS   SYSTEM    OF    MAN. 


45 


There  are,  however,  many  kinds  of  aphasia.  The 
one  above  mentioned  is  motor  aphasia.  But  there  is  also 
an  auditory  aphasia  {a.ap/i),  in  which  the  patient  can 
speak  but  can  not  understand  spoken  words ;  a  visual 
aphasia  {v.aph),  in  which  the  patient  can  not  read  ;  and, 
finally,  a  graphic  aphasia  {g.aph),  in  which  the  patient 
can  not  write.  These  are  situated  in  different  parts  of 
the  brain  and  shown  on  Fig.  29.* 

The  higher  operations  of  the  mind,  such  as  self- 
consciousness,  thought,  moral  sentiment,  etc.,  which 
the  older  phrenology  sought  to  locate,  are  possibly  not 
localized  at  all,  but  involve  the  co-operative  activity  of 
the  whole  brain,  and  such  co-operative  activity  is  prob- 
ably controlled  by  special  centers  of  association  yet  un- 
known.f 

Dextrality. — We  have  said  that  aphasia  is  an  affec- 
tion of  a  certain  convolution  of  the  frontal  lobe,  espe- 
cially on  the  left  side.  This  naturally  leads  one  to  draw 
attention  to  the  fact  that  the  cerebral  hemispheres- 
control  each  the  opposite  side  of  the  body.  The  fibers 
from  the  gray  matter  of  the  cortex  coming  down  cross 
over  to  the  other  side.  It  would  seem,  therefore,  that 
the  greater  dexterity  of  the  right  side — right-sidedness 
— is  the  result  of  the  higher  development  of  the  left 
cerebral  hemisphere — left-brainedness.  Dexterity  is  a 
more  perfect  co-ordination  of  muscular  motion.  Now, 
there  is  nothing  in  which  this  is  more  conspicuous  than 
in  speech. 

SECTION    II. 
Spinal  Cord. 

We  have  already  said  that  the  basal  part  of  the  brain 
may  be  regarded  as  an  intercranial  continuation  of  the 

*  Duval,  Rev.  Sci.,  vol.  xl,  p.  769,  1S87. 

f  Turner,  Brit.  Assoc.  Address,  Nature,  vol.  Ivi,  p.  525,  1897. 


46      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

spinal  cord,  or,  conversely,  the  spinal  cord  as  an  extra- 
cranial continuation  of  the  basal  part  of  the  brain. 
But  this  extracranial  part  is  so  important  that  it  must  be 
treated  separately. 

Envelopes. — Like  the  brain,  it  is  incased  and  pro- 
tected by  a  bony  cover  ;  but  in  this  case  the  bony  cover 
must  be  flexible,  and  is  therefore  jointed.  This  is  the 
jointed  backbone  so  characteristic  of  vertebrate  ani- 
mals, and  giving  name  to  the  department.  Like  the 
brain,  also,  it  is  invested  by  membranes — an  outer  tough 
fibrous  and  a  thin  vascular  one.  As  in  the  brain,  too, 
it  is  the  inflammation  of  the  membranes  which  gives  rise 
to  the  acuter  forms  of  disease,  such  as  cerebro-spinal 
meningitis. 

Description. — The  spinal  cord  is  a  nearly  cylindrical 
white  cord,  about  half  an  inch  in  diameter  and  eighteen 
inches  long.  Like  the  brain,  it  is  a  double  organ,  divided 
almost  into  two  semicylinders  by  a  cleft  down  the  dorsal 
and  the  ventral  side.  Thus  it  consists  of  two  semicylin- 
ders joined  along  the  axis.  Therefore,  viewed  from  the 
ventral  side,  we  have  two  anterior  columns,  and,  from  the 
dorsal  side,  two  posterior  columns.  The  posterior  col- 
umns carry  sensory  fibers;  the  anterior,  motor  fibers. 

Spinal  Nerves. — From  the  spinal  cord  proper  there 
go  off  thirty-one  pairs  of  nerves;  from  the  intercranial 
continuation  of  the  same  there  go  off,  in  addition,  twelve 
pairs — making,  in  all,  forty-three  pairs  of  axial  nerves. 
The  spinal  or  extracranial  (but  not  the  intercranial) 
nerves  have  each  two  roots,  which  quickly  unite  to  form 
one  nerve.  One  of  these  roots  is  connected  with  the 
posterior  or  sensory  column,  and  one  with  the  anterior 
or  motor  column  of  the  cord.  The  posterior  root  has  on 
it  a  knot  or  ganglion  (Fig.  30,  a,  b,  and  c).  These  nerves 
pass  out  between  the  joints  of  the  backbone  and  go  to 
be  distributed  to  all  parts  of  the  body.     This  is  the  case 


THE    NERVOUS   SYSTEM    OF    MAN. 


47 


until  we  reach  nearly  to  the  sacrum,  where  the  cord 
splits  up  at  once  into  nerves,  but  still  in  pairs,  to  form 
the  Cauda  equincB — horsetail  (Fig.  30,  d). 


Fig.  30. — Spinal  cord  :  «,   showing  the  membrane ;   b,  the  two  roots  ;   c, 
transverse  section  showing  the  two  roots  ;  </,  cauda  equinae. 

Section. — On  making  a  transverse  section  (Fig.  30,  b), 
it  is  at  once  seen  that  the  two  kinds  of  nerve  matter  are 
found  here  also.  But  here,  as  in  the  intercranial  basal 
continuation  (but  not  in  the  cerebrum  and  cerebellum), 
the  gray  matter  is  within,  and  the  white  matter  on  the 
outside  inclosing  it.  We  see  also  that  the  gray  matter 
has  a  peculiar  form,  found  also  in  the  medulla — viz.. 
that  of  a  semicircle  on  each  side  and  a  connecting  band 
between  (Fig.  30,  h).  As  this  is  only  a  section  view,  it 
is  evident  that  the  gray  matter  on  each  side  is  in  the 
form  of  a  plate  scrolled  outward  and  connected  by  a  flat 
5 


48      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

transverse  plate.  Tracing  the  fibers  from  the  nerves, 
we  find  that  those  from  the  posterior  roots  are  seen  to 
enter  the  posterior  horn  of  the  gray  matter;  and  those 
of  the  anterior  roots,  into  the  anterior  horn  of  the  gray 
matter  (Fig.  31).  The  former  enter,  each,  a  sensory  cell, 
and  through  it  communicates  with  a  fiber  going  up  the 
posterior  column  toward  the  brain,  and  even,  perhaps. 


Fig.  31. — '"ross  section  of  half  the 
spinal  crd,  showing:  how  srnsory 
fibers,  s,  and  motor  fibers,  f/i, 
enter  the  gray  matter. 


Fig.  ^2. — Cross  and  longitudinal 
section,  showing  how  sensory 
fibers,  s,  and  motor  fibers,  m, 
communicate  with  cells  of  gray 
matter  and  pass  on.  Arrows 
show  direction  of  transmission. 


to  the  cerebral  gray  matter  ;  the  latter  similarly  through 
a  motor  cell  and  upward  perhaps  to  the  cerebrum. 
This  is  shown  in  section  (Fig.  32). 

Geiural  Functioti. — The  general  function  of  the  cord, 
therefore,  is  twofold  :  first,  it  is  a  cable — the  biggest 
cable— of  conducting  fibers  from  the  gray  matter  of 
the  brain  to  all  parts  of  the  body;  and,  second,  it  is 
also  a  center  of  gray  matter  from  which  issue  fibers  to 
all  parts.  As  a  center,  its  distinctive  function  is  reflex 
or  automatic.  It  is  called  reflex  because  an  impression 
coming  from  a  sensitive  surface  to  this  center  is  reflected 


THE    NERVOUS   SYSTEM    OF    MAN. 


49 


back,  like  a  bounding  ball,  to  the  appropriate  muscle 
without  rising  into  consciousness  at  all.  To.it  is  as- 
signed the  control  of  all  those  wholly  unconscious  and 
involuntary  movements — such  as  those  of  the  heart,  the 
stomach,  and  the  intestines — so  necessary  to  the  con- 
tinuance of  bodily  life. 

The  gray  matter  of  the  cord  is  continuous  with  the 
gray  matter  of  the  basal  part  of  the  brain,  especially  of 
the  medulla,  but  entirely  separated  from  the  e.xterior 
gray  matter  of  the  cerebrum  and  cerebellum.  The 
function  of  the  gray  matter  of  the  medulla,  like  that  of 
the  spinal  cord,  is  wholly  reflex  or  automatic;  that  of 
the  thalamus,  semiautomatic.  Thus,  speaking  generally, 
as  we  go  headward  and  cerebrumward  the  function  of 
gray  matter  becomes  higher,  less  automatic,  more  dis- 
tinctly conscious,  and  voluntary. 

SECTION    III. 

Nerves. 

We  have  already  said  that  there  are  forty-three  pairs 
of  axial  nerves — i.  e.,  twelve  intercranial  and  thirty-one 
extracranial  or  spinal. 

Cranial  Nerves. — These  all  come  from  the  inter- 
cranial continuation  of  the  axis  at  the  base  of  the  brain, 
and  not  from  the  great  outgrowths  of  the  cerebrum  and 
cerebellum,  unless  we  except  one,  the  olfactory,  coming 
from  the  cerebrum.  They  all  pass  through  holes  in  the 
base  of  the  skull,  and  are  distributed,  with  two  excep- 
tions, to  the  head  and  face.  Several  of  them  are  nerves 
of  special  sense.  The  cranial  nerves,  on  account  of 
their  higher  and  more  differentiated  functions,  have 
all  of  them,  in  addition  to  their  ordinal,  also  special 
names.  In  the  order  of  their  position,  beginning  in 
front,  they  are  : 


50 


PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 


First  pair. — Olfactory  =  Special  sense  of  smell. 

Second  pair. — Optic  =  Special  sense  of  sight. 

Third.,  fourth.,  and  sixth  pairs. — Motors  of  the  eye. 

Fifth  pair. — Trigemini  =  Common  sensory  of  the  face. 

Seventh  pair. — Facial  =  Common  motor  of  the  face. 

Eighth  pair . — Auditory  =  Special  sense  of  hearing. 

Ninth  pair. — Glosso-pharyngeal  (gustatory)  =  Special 
sense  of  taste. 

Tenth  pair. — Vagus  or  pneumogastric  =  Sensory  and 
motor  of  the  heart,  lungs,  and  stomach. 

Eleventh  pair. — Recurrens  =  Motor. 

Tioelfth pair. — Hypoglossal  =  Motor  of  the  tongue. 

In  Fig.  ^Tf  we  give  a  diagram  showing  general  distri- 
bution of  these  nerves.  The  origins,  as  seen  in  Fig.  17, 
page  2,2,,  are  near  together.  In  this  Fig.  33  the  base  of 
the  brain,  and  especially  the  medulla,  are  drawn  out  so 
as  to  separate  these  origins.  In  Fig.  34  the  medulla  is  a 
little  drawn  out.  Fig.  17,  page  2,3,  shows  their  natural 
position. 

Description. — i.  The  olfactory  nerve,  the  only  one 
which  comes  from  the  cerebrum,  is  observed  to  pass  for- 
ward beneath  the  frontal  lobe  (Fig.  17),  on  the  base  of 
the  skull,  until  it  reaches  just  above  the  nasal  cavity 
and  on  each  side  of  the  crista  galli.  There  it  throws 
out  a  great  number  of  small  branches  (Fig.  t,?);  0  through 
the  cribriform  (colandered)  plate,  to  be  distributed  to  the 
mucous  membrane  of  the  upper  cavities  of  the  nostrils. 
Its  function  is  to  respond  to  the  impression  of  odorifer- 
ous vapors. 

Even  in  man  it  is  seen  to  swell  a  little  at  the  end. 
As  we  pass  down  the  vertebrate  scale,  by  the  increasing 
size  of  this  swelling  it  loses  entirely  its  character  as  a 
nerve  and  becomes  a  great  lobe — the  olfactory  lobe  of 
the  brain. 

2.   Optic  Nerve. — As  already  said  (page  41),  this  arises 


52 


PHYSIOLOGY   AND    MORPHOLOGY   OP^    ANLMALS. 


by  two  roots  on  each  side,  one  from  the  optic  lobe  and 
one  from  the  thalamus.  These  quickly  unite  to  form 
one  root  on  each  side,  and  these  join  to  form  the 
optic  chiasm,  which  lies  on  the  sella  turcica  (Turkish 
seat).  This  is  all  within  the  skull  (see  Figs.  17  and  ;^^). 
The  chiasm  immediately  separates  again  into  two  large 
nerves,  the  optic  nerves  proper,  which,  piercing  the  skull 


Fig.  34. — Diagram  showing  side  view  of  brain  and  medulla  somewhat 
drawn  out  to  separate  the  origin  of  the  nerves.  The  last  one  (i  s/>)  is 
the  first  spinal. 


at  the  bottom  of  the  eye  sockets,  pass  forward  to  enter 
the  eyeball,  and  there  form  the  retina.  Its  function  is, 
of  course,  to  respond  to  impressions  of  light. 

3,  4,  and  6.  Ocu/i  Motores. — These  we  take  together 
because  they  all  have  a  somewhat  similar  function,  viz., 
the  movements  of  the  eyeball.     They  come  out  from  the 


THE    NERVOUS   SYSTEM    OF    MAN. 


53 


anterior  part  of  the  medulla,  and  are  distributed  to  the 
ocular  muscles  (Figs.  ;^^  and  34). 

5.  Trigeminal. — This  comes  from  the  anterior  portion 
of  the  medulla,  pierces  the  skull,  and  comes  out  on  the 
face  on  each  side,  just  in  front  of  the  ear.  It  forms 
there  a  ganglion  or  knob,  and  then  divides  into  three 
branches  and  is  distributed  to  all  parts  of  the  face  to 
form  the  nerves  of  sensation  of  the  face.  It  is  a  morbid 
condition  of  this  nerve  which  constitutes  neuralgia  of 
the  face,  or  tic  douloureux.  Fig.  34  shows  how  a  branch 
of  this  nerve  goes  to  each  tooth.  Toothache  also  is  a 
painful  affection  of  this  nerve. 

7.  Facial. — This,  also  originating  from  the  medulla, 
comes  out  on  the  face  near  the  ear  and  ramifies  over 
the  whole  face  and  head.  It  is  the  general  motor  nerve 
of  the  face.  It  controls  all  the  facial  muscles,  and 
therefore  gives  emotional  expression.  Paralysis  of  the 
face  IS  an  affection  of  this  nerve. 

8.  Auditive. — Coming  also  from  the  medulla  in  close 
connection  with  the  last,  this  does  not  come  out  on  the 
face  at  all,  but  passes  immediately  into  the  inner  ear, 
to  be  distributed  there  as  the  nerve  of  hearing. 

9.  Glossopharyngeal  [Gustatory). — It  is  not  quite  cer- 
tain what  nerve  is  the  gustatory,  but  the  distribution  of 
this  one  to  the  back  part  of  the  tongue  and  adjacent 
parts  of  the  throat,  where  the  gustatory  sense  chiefly 
resides,  makes  it  probable  that  this  is  it.  The  distribu- 
tion is  shown  in  Fig.  ^i,  page  51. 

10.  Vagus  or  Pneumogastric. — This  large  nerve  comes 
from  the  medulla,  passes  through  the  base  of  the  skull 
and  down  into  the  thoracic  and  abdominal  cavities,  and 
is  distributed  to  the  lungs,  the  heart,  and  the  stomach. 
It  reports  their  condition  and  wants  and  determines 
their  movements.  It  is  therefore  both  sensory  and 
motor. 


54      PHYSIOLOGY    AND    MORPHOLOGY   OF    ANIMALS. 

11.  Spinal  Recurrent. — So  called  because,  arising  from 
the  spinal  cord  outside  of  the  cranium,  it  passes  upward 
within  the  backbone,  enters  the  skull,  and  again  comes 
out  to  be  distributed  to  the  muscles  of  the  shoulder.  It 
is  a  motor  nerve. 

12.  Hypoglossal. — Arising  from  the  medulla,  low 
down,  just  before  it  becomes  the  cord,  it  passes  out  to 
be  distributed  to  the  tongue  and  to  become  its  motor 
nerve.     It  therefore  controls  articulation. 

General  Observations  on  Cranial  Nerves. — Observe  (i) 
they  all  except  No.  i  come  from  the  base  of  the  brain 
or  intercranial  continuation  of  the  axis;  (2)  all  except 
I  and  2  come  from  the  medulla ;  (3)  all  the  special 
senses  are  to  be  found  here;  (4)  in  most  cases  the  sen- 
sory and  motor  fibers  are  embodied  in  separate  nerves, 
in  this  regard  differing  from  the  spinal  nerves,  which 
have  each  two  roots,  a  sensory  and  a  motor. 

Spinal  Nerves. — As  already  said,  there  are  of  these 
thirty-one  pairs,  each  with  its  two  roots.  Their  dis- 
tinctive functions  are  not  so  different  as  in  the  case  of 
the  cranial,  and  they  do  not  therefore  need  distinct 
names.  They  are  divided  into  four  groups  :  cervical, 
dorsal,  lumbar,  and  sacral.  There  are  eight  cervical, 
twelve  dorsal,  five  lumbar,  and  six  sacral.  Those  of  each 
group  are  numbered  first,  second,  third,  etc.  (Fig.  35). 

Distribution. — Most  of  these  are  distributed  to  adja- 
cent parts  of  the  body,  but  in  the  upper  and  lower  por- 
tion of  the  series  several  are  united  to  form  the  great 
limb  nerves.  Thus  the  fifth,  sixth,  seventh,  and  eighth 
cervical  and  first  dorsal  form  a  plexus  from  which  go  the 
nerves  of  the  arm  and  hand,  while  the  two  last  lumbars 
and  four  of  the  sacrals  form  the  plexus  from  which  go 
the  great  nerves  which  supply  the  leg  and  foot.  In  all 
cases  by  division  and  subdivision  the  branches  become 
smaller  and  smaller  until   they  pass  beyond  the  power 


THE    NERV'OUS   SVSTEM    OF    MAN. 


33 


D-l 


I^-.^ 


^  'G.  35.— Diag:ram  showing  spinal  nerves  and  their  distribution  :  c,  i,  2,  3, 
etc.,  cervical ;  (/,  i,  2,  3,  etc.,  dorsal ;  /,  i,  2,  3,  etc.,  lumbar  ;  J,  i,  2  3. 
etc.,  sacral.     After  Flower. 


56      PHYSIOLOGY   AND   MORPHOLOGY   OF   ANLMALS. 

of  naked-eye  vision,  and  finally  terminate  mainly  in  two 
ways,  viz.,  some  in  ?niiscuiar  tissues  and  some  in  sefisitive 
surfaces  and  sense  organs. 

Structure  of  Nerves. — A  nerve  is  a  bundle  of 
slender  fibers  of  extreme  fineness  lying  parallel  and  in- 
vested by  a  membrane  of  fibrous  tissue — neurolemma. 
The  size  of  the  fibers  varies  from  y^Vo  ^'^  ^  oooo  °^  ^"^ 
inch,  or  even  less.  The  coarsest  are  the  motor  fibers 
and  the  finest  the  sensory  fibers  of  the  optic  nerve.  The 
number  in  a  nerve  of  Jg  inch  in  diameter  may  be  a  mil- 
lion or  more.  Each  fiber  consists  of  a  central  medullary 
part  and  an  investing  sheath.  Each  fiber  may  be  con- 
tinuous from  a  cell  in  the  central  gray  matter  to  its  ter- 
mination in  the  tissue,  but  this  is  probably  not  true  of  all. 
The  cerebral  cells  connect  with  the  surface  only  through 
a  chain  of  several  cells  in  the  thalamus,  the  medulla,  and 
the  spinal  column.  A  branch  of  a  nerve  therefore  con- 
sists of  a  number  of  fibers  separated  and  invested  as 
before,  but  without  branching  of  the  fibers  themselves, 
except  at  the  extreme  end  where  they  may  form  dendrites. 
Thus  we  may  regard  each  fiber  as  continuous,  one  end 
terminating  in  a  central  cell,  the  other  in  a  tissue. 

Function  of  Nerves. — Nerve  fibers  are  of  two 
kinds,  sensory  and  motor.  The  one  transmits  external 
impressions  inward  to  the  nerve  center  (afferent),  and  may 
or  may  not  awaken  consciousness;  the  other  transmits 
internal  impulses  on fraard  (efferent),  and  determines  mus- 
cular contraction.  These  two  kinds  may  lie  side  by  side 
in  the  same  nerve  undistinguishable  from  one  another 
except  that  the  motor  is  usually  larger.  The  termina- 
tions of  the  one  are  centrally  in  a  sensory  cell  of  the 
central  gray  matter,  and  peripherally  by  a  peculiar  end- 
ing in  a  sensitive  surface  or  a  sense  organ  ;  the  termina- 
tions of  the  others  are  centrally  in  a  motor  cell  of  the 
central  gray  matter,  and  peripherally  in  a  muscular  fiber. 


THE    NERVOUS   SYSTEM    OF    MAN.  57 

Every  mechanism  for  action  and  reaction  between 
the  organism  and  the  external  world  must  consist  of 
two  kinds  of  central  cells  (a  sensory  and  a  motor),  two 
kinds  of  transmitting  fibers  (afferent  and  efferent),  and 
two  kinds  of  peripheral  terminations  (a  sensitive  sur- 
face or  sense  organ  and  a  muscular  or  contractile  tissue). 
Fig-  36,  repeated  from  page  26,  is  a  diagram  illustrating 
this  action  and  reaction.  The  manner  in  which  the 
whole  acts  is  briefly  as  follows:  Impression  on  a  sen- 
sitive surface  or  sense  organ  (S)  is  transmitted  cen- 
tripetally  along  a  sensory  fiber  to  the  nerve  center, 
awakens   response,  which    is    transmitted    centrifugally 


^S2 


Fig.  36. — Diagram  showing  essential  parts  of  an  apparatus  of  exchange  be- 
tween the  external  world  and  consciousness  :  A'C,  ner\-e  center  ;  sc,  sen- 
sory cell ;  sf,  sensory  6ber ;  SS,  sensory  surface  ;  mc,  motor  cell ;  /«/", 
motor  fiber ;  M,  muscle.  Arrowheads  show  the  direction  of  transmis- 
sion. 

along  a  motor  fiber,  and  determines  muscular  contrac- 
tion, which  produces  motion. 

Both  these  kinds  of  fibers  are  inclosed  in  the  same 
sheath  in  the  case  of  spinal  nerves,  but  are  usually  sepa- 
rated and  found  in  different  nerves  in  the  case  of  the 
cranial  nerves.  Thus  in  the  case  of  the  cranial  series 
we  speak  of  sensory  and  motor  nerves,  but  in  the  spinal 
series  we  can  only  speak  of  sensory  and  motor  ^^ers. 

The  Two  Subsystems. — The  cerebro-spinal  system  may 
be  conveniently  subdivided  into  two  subsystems.  By 
function  they  may  be  called  the  conscio-voluntary  and  the 


58 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 


reflex;  by  center  they  may  be  called  cerebral  ^\\^  spinal 
or  axial,  for  it  includes  the  medulla  as  well  as  the  cord. 
The  center  of  the  one  is  the  surface  gray  matter  of  the 
cerebrum  ;  the  center  of  the  other  the  central  gray  matter 
of  the  cord  ■d.x\^  its  continuation  in  the  skull.  Each  sub- 
system has  its  sensory  or  afferent  and  its  motor  or  ef- 
ferent fibers,  but  the  two  subsystems  are  so  closely 
connected  that  they  may  act  as  one.  The  spinal  nerves 
carry  both  kinds  of  fibers,  which  may  act  as  belonging 
to  both  systems.  The  cranial  nerves  usually  carry  but 
one  kind — i.  e.,  either  sensory  or  motor,  acting  for  both 
systems. 

Course  and  Termination  of  Fibers. — A  sensory  fiber  of 
the  cerebral  system,  beginning  in  a  sensory  cell  of  the 
cerebral  cortex,  passes  down  a  posterior  column  of  the 
cord,  communicates  with  a  sensory  cell  of  a  posterior 
cornu,  and,  continuing,  becomes  a  sensory  fiber  of  the 
reflex  system  as  well  as  the  cerebral  system,  and  then 
goes  out  by  a  posterior  root  of  a  spinal  nerve  to  termi- 
nate in  a  sensitive  surface  or  a  sense  organ.  A  motor 
fiber  of  the  same  system  goes  from  a  motor  cell  of  the 
cerebral  cortex,  down  an  anterior  column  of  the  cord, 
communicates  with  a  motor  cell  of  the  anterior  cornu, 
and  continues  as  a  motor  fiber  of  both  systems,  to  termi- 
nate in  a  muscle.  The  sensory  and  motor  cells,  both  of 
the  cerebrum  and  of  the  spinal  cord,  connect  with  one 
another,  so  as  to  complete  the  circuit,  of  the  cerebral 
system  in  the  one  case,  and  of  the  spinal  system  in  the 
other. 

Or,  more  explicitly,  and  tracing  each  impulse  in  the 
direction  of  its  transmission  :  .\  sensory  fiber  of  the 
cerebral  system,  commencing  in  a  terminal  on  a  sensi- 
tive surface  or  in  a  sense  organ,  passes  up  a  spinal 
nerve,  through  a  posterior  root  into  a  posterior  cornu, 
communicates  there  with  a  spinal  sensory  cell,  then  goes 


THE    NERVOUS   SYSTEM    OF    MAN. 


59 


C?7h 


up  a  posterior  column  to  the  thalamus,  communicating 
with  a  sensory  cell  of  that  ganglion,  and  thence  onward 
to  a  sensory  cell  of  the 
cerebral  cortex,  awaken- 
ing consciousness  there  ; 
then  the  impression  is 
transferred  to  a  motor  cell 
of  the  cerebral  cortex, 
which  sends  it  on  in  the 
form  of  will  down  through 
a  motor  cell  of  the  corpus 
striatum,  then  down  a  fiber 
of  an  anterior  column  of 
the  cord,  and,  after  com- 
municating with  a  spinal 
motor  cell,  out  by  an  an- 
terior root  and  a  spinal 
nerve,  to  terminate  in  a 
muscle  and  cause  contrac- 
tion there. 

In  the  reflex  system 
the  course  is  the  same,  ex- 
cept  that   the  impression 

carried  up  by  the  sensory    Fig.  ,^7.     Diagram  of  brain,  thalamus- 

.         .     ,,  corpus,  and  a  portion  of  spinal  cord, 

Xxh^X"" short  Circuits      across  representing:  course  of  transmission 

f-^,^     »k„    ^,,:.,„i     r.^^-. of  nerve  influence:  cj,  cerebral  sen- 

from     the    spinal     sensory  sory,  and  .;«,  cerebral  motor  cells ; 

cell    to     the    spinal    motor  J/j,  spinal  sensory,  and  j/w,  spinal 

.  motor   cells  ;  jj,    sensitive    surface  ; 

cell     without    going    up    to  m,   muscle.     The  arrows  show   the 

.1  u  i  1  direction  of  transmission. 

the  cerebrum    to   awaken 

consciousness  there.     The  course  is  shown    in   the  dia- 
gram (Fig.  37). 

General  Mode  of  Action  of  the  Whole. — Suppose  each 
sensory  fiber  to  have  its  own  terminal,  its  own  spinal 
sensory  cell,  and  its  own  cerebral  sensory  cell,  and  each 
motor  fiber  to  have  its  own  muscular  fiber  terminal,  its 


6o      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

own  spinal  motor  cell  and  cerebral  motor  cell,  and  the 
sensory  cells  in  the  cerebrum  and  in  the  cord  to  com- 
municate each  with  its  corresponding  motor  cell.  Thus, 
every  cerebral  sensory  cell  would  have  its  correspond- 
ing terminal  on  the  body  surface,  with  a  connecting  fiber 
between,  passing  through  a  corresponding  spinal  sen- 
sory cell;  and  every  cerebral  motor  cell  its  correspond- 
ing spinal  motor  cell  and  muscular  terminal  with  a  con- 
necting fiber  between.  Now  touch  a  point  on  the  body 
surface,  and  a  wave  or  current  of  influence  is  carried 
along  a  sensory  fiber  through  the  cord,  as  already  ex- 
plained, to  a  cerebral  sensory  cell,  awakening  conscious- 
ness there.  Immediately,  or  perhaps  only  after  delib- 
eration, the  influence  is  transferred  by  a  connecting 
fiber  to  a  cerebral  motor  cell,  awakening  will,  and  by 
It  down  the  spinal  cord  by  a  motor  fiber,  and  out  to  a 
muscle  determining  appropriate  motion. 

See,  then,  all  the  phenomena  in  a  case  of  simple  re- 
sponse to  external  impression  :  (i)  Impression  ;  (2)  trans- 
mission inward  ;  (3)  change  in  a  sensory  cell — conscious 
sensation;  (4)  transmission  to  a  motor  cell;  (5)  change 
in  the  motor  cell — luill \  (6)  transmission  outward  along 
a  motor  fiber  ;  (7)  contraction  of  a  muscle.  Metaphor- 
ically we  might  say  that  we  have  here  a  complex  instru- 
ment of  communication  between  the  external  world  and 
the  conscious  self,  with  the  self  playing  on  brain  cells  or 
interior  nerve  terminals  at  one  end,  and  the  external  world 
playing  on  exterior  nerve  terminals  at  the  other  end. 

In  Reflex  Action. — If  the  impression  is  on  an  interior 
surface  in  a  normal  condition,  the  current  of  influence 
on  reaching  the  spinal  sensory  cell  is  transferred  across 
by  short  circuit  to  the  corresponding  spinal  motor  cell 
and  reflected  immediately  back  along  a  corresponding 
motor  fiber  to  the  appropriate  muscle,  without  rising  at 
all  into  consciousness.     Such  is  the  case  in  impressions 


THE   NERVOUS   SYSTEM    OF    MAN.  6l 

on  the  stomach,  heart,  etc.  In  other  cases,  as  in  swal- 
lowing, sneezing,  coughing,  breathing,  etc.,  the  current 
short  circuits,  indeed,  and  appropriate  motion  takes 
place  immediately  by  reflex,  but  sufficient  overflow 
reaches  the  cerebrum  to  produce  consciousness.  In 
ordinary  cases  of  impression  on  an  external  consciously 
sensitive  surface  or  sense  organ,  as  already  seen,  the  cur- 
rent passes  on  without  short  circuit  directly  to  the  cere- 
brum, and  consciousness  takes  charge  of  the  response; 
but  if  the  impression  be  painful,  then  the  current  short 
circuits  without  waiting  for  the  slower  action  of  the 
cerebral  system. 

For  simplicity's  sake  we  have  represented  the  connec- 
tion throughout  as  physical  and  continuous;  but,  as 
already  explained  (Fig.  21,  page  37,  Fig.  28,  page  42), 
the  connection  between  neurones  is  probably  by  touch- 
ing fingers  or  interlacing  dendrites.  It  has  been  sug- 
gested that  the  fingerlike  extensions  are  like  pseudopods 
of  amoebie — that  by  extension  and  contraction  they  make 
and  break  contact  with  one  another.  In  the  active  wak- 
ing state  they  elongate  and  make  contact;  in  uncon- 
sciousness, in  coma,  and  in  sleep  they  contract  and  break 
contact.  On  this  view  disconnection  of  neurones  is  the 
physical  cause  of  sleep.* 

Illustration  by  Telegraphy. — To  enforce  these  princi- 
ples still  further  and  make  them  still  clearer  we  make  a 
somewhat  elaborate  comparison  with  a  system  of  teleg- 
raphy. 

Suppose,  then,  the  Capitol  at  Washington  represents 
\.\\^  head.  In  it  there  is  a  great  rotunda;  this  represents 
the  cerebrum.  Suppose  all  about  the  walls  a  series  of 
alcoves;  these  shall  be  the  convolutions.  These  are,  say, 
full  of  battery  cells;  these  are  the  sensory  and  motor 

*  Mathias  Duval,  Rev.  Sci.,  i.x,  321,  1898. 


62      PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

cells  of  the  cerebrum.  From  these  battery  cells  there 
go  wires,  converging  to  the  hallway  and  forming  there 
a  great  cable  of  wires  going  out  of  the  door  ;  these  are 
the  white  fibers  converging  and  forming  the  w^(//^//c?,  and 
going  out  of  the  skull  as  the  cord.  Before  going  out, 
however,  certain  wires  are  sent  out  from  the  cable  to 
all  the  offices  in  the  building;  these  are  the  cranial 
nerves  going  to  the  head  and  face,  and  especially  to  the 
sense  organs.  The  cable  starts  out  now  to  communi- 
cate with  the  whole  country,  but  protected  by  an  arch- 
way ;  this  is  the  cord  protected  by  the  vertebral  column. 
As  it  goes,  the  cable  gives  out  wires  to  adjacent  and 
even  distant  regions  ;  these  are  the  spinal  nerves.  These 
must  go  to  every  State,  county,  and  city,  and  terminate 
in  intelligence  offices  and  in  executive  or  police  offices; 
these  are  the  sense  organs  and  the  muscles.  Suppose 
also  the  alcoves  are  all  named  as  States  and  the  bat- 
teries all  numbered. 

Now,  suppose  anything  to  occur  in  any  place.  The 
intelligence  office  reports  the  fact  to  the  head  center. 
The  State,  county,  city,  neighborhood,  is  at  once  known, 
and  the  command  immediately  goes  out  to  the  execu- 
tive office  and  determines  appropriate  action. 

Application. — Let  us  now  apply  this  idea  and  show 
how  it  explains  the  j)henomena  : 

1.  Cut  the  cord  high  up  in  the  neck.  The  whole  body 
is  paralyzed  to  both  consciousness  and  volition,  but  not 
to  reflex  function,  for  that  is  in  the  gray  matter  of  the 
cord,  which  we  are  not  now  considering.  Prick  the  foot 
and  it  will  jerk,  but  the  prick  is  unfelt  and  the  jerk  is  in- 
voluntary. Meanwhile  all  the  parts  of  the  face  are  unpar- 
alyzed.  The  patient  sees  and  speaks  as  usual,  because 
the  nerves  controlling  these  come  out  from  the  medulla. 

2.  Cut  the  cord  in  the  middle  of  the  back.  Now  the 
upper  parts  of  the  body,  including  the  arms,  etc.,  feel 


THE    NERVOUS   SYSTEM    OF    MAN.  63 

consciously  and  may  be  moved  voluntarily;  but  the 
whole  lower  portion,  including  the  legs,  is  paralyzed 
both  to  conscious  sensation  and  to  voluntary  motion, 
because  these  parts  are  cut  off  from  the  cerebral  center. 
But  reflex  movements  remain. 

3.  Cut  the  posterior  root  of  a  spinal  nerve.  Now 
all  that  part  to  which  this  nerve  is  distributed  is  para- 
lyzed to  sensation,  but  may  be  voluntarily  moved.  If, 
on  the  contrary,  the  anterior  root  is  cut  instead  of  the 
posterior,  then  the  part  is  paralyzed  to  motion,  though 
not  to  sensation.  If,  finally,  the  nerve  is  cut  below  the 
junction  of  the  two  roots,  then  the  part  to  which  the 
nerve  is  distributed  is  paralyzed  to  both  sensation  and 
motion. 

4.  Irritate  a  nerve  in  its  course — say,  by  pinching  it. 
For  example,  pinch  or  strike  the  ulnar  nerve,  lying  be- 
tween the  elbow  joint  and  the  inner  condyle.  We  are 
all  familiar  with  the  fact  that  we.  feel  pain  in  the  little  and 
ring  fingers,  where  this  nerve  is  distributed.  If  it  were  not 
for  the  skin  covering  the  nerve,  and  which  of  course 
has  its  own  nerves  of  sensation — if  the  skin  w'ere  cut 
away  so  as  to  bare  the  nerve  and  the  nerve  alone  was 
pinched,  the  only  sensation  we  should  feel  would  be  in 
the  little  and  ring  fingers  and  that  side  of  the  hand. 
Why  ?  Because  the  nerves  are  distributed  there.  The 
intelligence  ofifices  are  there.  Therefore  at  the  head 
center  the  painful  intelligence  seems  to  be  reported  from  there. 
How  could  it  be  otherwise  ? 

5.  Cut  a  nerve,  perhaps  high  up  in  the  arm  or  leg. 
Expose  the  ends.  Pinch  the  end  below  the  cut ;  you 
feel  nothing.  But  pinch  the  end  above  the  cut ;  you 
feel  pain — but  where  ?  Not  at  the  place  pinched,  but 
in  the  fingers  or  toes  where  the  cut  nerve  is  distributed 
— i.  e.,  where  the  nerve  terminals,  the  intelligence  offices, 
are.     In  any  telegraphic  system,  if  a  wire  is  cut  and  a 


64      PHYSIOLOGY    AND    MORPHOLOGY   OF    ANIMALS. 

message  sent  from  the  cut  end,  the  head  office  at  Wash- 
ington could  not  but  refer  it  to  the  place  where  this 
wire  ought  to  go. 

Law  of  Peripheral  Reference. — Thus  we  have 
the  law  that  an  impulse  received  by  the  brain  through  a 
nerve  fiber  is  of  necessity  referred  by  the  consciousness  to  the 
peripheral  extremity.  .  .  .  This  explains  the  fact  that  in 
the  case  of  an  amputated  limb  the  patient  still  has  a 
sense  of  the  presence  of  a  foot  or  hand;  and  if  the 
nerve  of  the  stump  should  become  diseased,  he  will 
often  feel  intense  pain  in  foot  or  hand. 

Nerve  Force  versus  Electricity. — I  have  used 
this  comparison  with  a  telegraphic  system  in  order  to 
make  the  mode  of  action  of  the  nervous  system  clear. 
But  we  must  not  conclude,  therefore,  as  many  do,  that 
nerve  force,  and  indeed  life  itself,  is  nothing  but  elec- 
tricity. It  becomes  necessary,  therefore,  that  we  should 
draw  attention  to  some  fundamental  differences  be- 
tween these  two  forms  of  energy  : 

1.  Wires  lying  in  contact  with  one  another  in  the 
same  bundle  will  not  conduct  true  unless  insulated. 
Nerve  fibers,  on  the  contrary,  conduct  true  although 
lying  in  contact  in  the  same  sheath — in  a  moist  condi- 
tion, and  therefore  uninsulated. 

2.  Cut  a  wire  and  press  the  fresh-cut  ends  together 
— they  still  conduct  well.  But  a  cut  nerve  pressed  to- 
gether utterly  fails  to  conduct  nerve  influence. 

3.  In  the  case  of  an  electric  current  there  must  be 
a  closed  circuit.  This  is  fundamental.  If  the  circuit  is 
open  anywhere  there  is  no  current  and  can  not  be. 
Not  so  in  the  case  of  a  nerve  current.  There  is  indeed 
a  sensory  current  and  a  return  motor  current.  They 
are  connected,  too,  at  the  cerebral  end,  but  certainly 
not  at  the  peripheral  ends.  Besides,  there  is  often 
current   only   one   way — i.  e.,   sensation    without   corre- 


THE    NERVOUS   SYSTEM    OF    MAN.  65 

spending  motion,    or   motion    initiated   without   incitmg 
sensation. 

4.  The  velocity  of  electricity  is  always,  like  all 
ethereal  vibrations,  inconceivably  great;  but  the  veloc- 
ity of  a  nerve  current  has  been  measured  and  found  to 
be  very  moderate — only  about  one  hundred  feet  a  second. 
In  fact,  the  phenomena  of  transmission  of  nerve  influence 
would  suggest  an  analogy  with  propagated  chemical 
change,  such  as  combustion  of  a  train  of  gunpowder 
rather  than  electric  current. 

But  it  will  be  answered  that  "seeing  is  believing." 
The  electric  organ  of  certain  fishes,  as  the  electric  eel, 
discharges  powerful  currents — sufficient,  indeed,  to  kill 
a  man.  These  organs  are  connected  with  the  brain  by 
very  large  nerves.  The  discharge  of  electricity  is  cer- 
tainly under  the  control  of  the  will.  It  is  an  act  of 
volition.  The  fish  is  exhausted  by  it  as  by  any  power- 
ful effort. 

At  first  sight  this  seems,  indeed,  demonstrative  ;  but 
not  so.  All  the  forces  of  Nature,  nerve  force  and  life 
force  among  the  number,  are  correlated — i.  e.,  are  con- 
vertible one  into  another.  Now,  the  electric  organ  of 
a  fish  constitutes  an  arrangement  for  converting  nerve 
force  into  electricity,  precisely  as  a  muscle  is  an  arrange- 
ment for  converting  nerve  force  into  mechanical  power. 
^Ve  might  as  well  say  that  nerve  force  is  identical  with 
mechanical  power  as  to  say  that  it  is  naught  else  than 
electricity. 

The  fact  is,  there  are  many  different  forms  of  force 
in  Nature,  each  producing  a  peculiar  group  of  phenom- 
ena, the  study  of  which  gives  rise  to  a  peculiar  depart- 
ment of  science.  Now  the  phenomena  of  nerve  force 
are  so  different  from  those  of  electricity  that  these  two 
are  rightly  called  different /(?;'w.f  of  the  universal  energy, 
although,  indeed,  they  are  transmutable  into  one  another. 


66      PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 

Function  of  the  Spinal  or  Reflex  System. — As 

already  said,  the  function  of  this  system  is  to  preside 
over  and  control  all  the  routine  work  of  the  body — 
work  so  constantly  necessary  that  it  can  not  be  left  to 
the  conscio-voluntary  system,  which  is  occupied  with 
other  and  higher  work.  Thus  the  beating  of  the  heart, 
the  play  of^the  respiratory  muscles,  the  movements  of 
the  stomach  and  intestines,  are  under  the  control  of  this 
system,  which  never  sleeps  night  or  day*  The  conscio- 
voluntary  system  alone  sleeps.  The  passage  of  control 
from  one  system  to  the  other  is  well  seen  in  the  act  of 
swallowing.  The  food  is  chewed,  then  gathered  by  the 
tongue,  then  pressed  back  into  the  throat  ;  so  much  is 
under  control  of  the  voluntary  system.  As  soon  as  it 
touches  the  throat  it  is  seized  by  the  involuntary  sys- 
tem and  hurried  on  to  the  stomach.  Nevertheless,  there 
are  all  gradations  between  reflex  and  conscio-voluntary 
movements.  The  movements  of  stomach,  intestines,  and 
heart  are  not  only  involuntary,  but  also,  in  health,  un- 
conscious; the  acts  of  swallowing,  sneezing,  coughing, 
are  involuntary,  but  not  unconscious;  the  act  of  breath- 
ing is  not  only  conscious,  but  also  partly  controlled  by 
volition. 

When  the  conscio-voluntary  system  is  in  full  activity 
it  takes  possession  of  the  consciously  sensitive  surfaces 
and  the  voluntary  muscles,  so  that  the  reflex  system  is  in 
abeyance  except  under  conditions  of  extreme  stimula- 
tion or  pain,  in  which  case  the  reflex  takes  hold  because 
the  conscio-voluntary  is  too  slow.  But  when  the  conscio- 
voluntary  control  is  withdrawn,  as  in  sleep,  or  paralyzed, 
as  in  section  of  the  cord  or  of  a  nerve,  then  the  reflex  is 
far  more  active,  as  shown  by  the  unconscious,  involun- 
tary movements  of  hand  or  foot  on  the  least  irritation. 

Illustration  by  Telegraphy. — The  system  of  telegraphy 
already  used  to  illustrate  the  action  of  the  conscio-vol- 


THE    NEk\OUS   SYSTEM   OF    MAN.  67 

untary  system  may  be  made  to  illustrate  this  also  by 
adding  battery  cells  all  along  the  cable  within  the  arch- 
way, and  these  also  sending  out  wires  to  intelligence 
offices  and  executive  offices  in  every  part  of  the  country 
and  controlling  all  necessary  routine  business  without 
troubling  the  head  center  except  in  case  of  extreme 
emergency.  By  some  stretch  of  the  imagination  they 
may  be  compared  to  state  government. 


SECTION    IV. 
Ganglionic  System. 

It  will  be  remembered  that  we  divided  the  whole 
nervous  system  of  vertebrates  into  two  subsystems, 
viz.,  the  cerebro-spinal  and  the  ganglionic.  The  latter 
we  put  aside  for  the  time.  We  now  take  it  up,  but  very 
briefly,  because  it  is  very  imperfectly  understood. 

Definition. — Nerves  are  cylindrical  bundles  of  fibers. 
Every  knot  or  swelling  on  these  cylindrical  strings  con- 
tains gray  matter  with  cells  and  gives  out  the  two  kinds 
of  fibers  terminating  in  the  tissues.  In  a  word,  they  are 
little  centers  of  force  and  are  called  ganglia.  Now  the 
ganglionic  system  is  so  called  because  it  consists  en- 
tirely of  such  small  ganglia  scattered  about  in  the  body 
and  connected  by  nerve  strings. 

Description. — The  system  consists  (i)  of  a  series  of 
ganglia  on  each  side  of  the  spinal  column  (nt)t  in  the 
canal)  the  whole  way  from  the  base  of  the  skull  to  the 
end  of  the  sacrum,  one  opposite  each  joint  of  the  col- 
umn (see  Fig.  38,^^).  (2)  This  series  of  ganglia  is  con- 
nected throughout  on  each  side  by  a  nerve  cord.  The 
two  knotted  cords  thus  formed  are  called  the  sympa- 
thetic nerves  (Fig.  38,  //  //).  (3)  From  each  spinal  nerve 
there  goes  off  a  small  branch  which  connects  with  the 
sympathetic  nerve  on  each  side,  and   thus  the  two  sys- 


68      PHVSlULOGV    AND    MOKPIIOLOGV    OF    AMMALS. 


^\pl  * 


Fig.  38.  —  Diaf^ram  showinp  distribution  of  the  {^aii;  lionic  system  :  g^  {jan- 
glions  ;  «,  sympathetic  nerve  ;  csp,  connectinj;  spinal  branch  ;  //,  1,  2,  3, 
etc.,  plexuses. 


THE    NERVOUS   SYSTEM    OF    MAN.  69 

terns,  the  axial  and  the  ganghonic,  are  brought  into  re- 
lation with  one  another  (Fig.  38,  en  01).  (4)  From  the 
sympathetic  gangha  on  each  side  there  go  nerves  to  the 
visceral  region,  where  are  performed  the  most  impor- 
tant functions.  There  the  nerves  from  each  side  unite 
to  form  plexuses  or  networks — i.  e.,  the  nerves  cross  one 
another  in  every  direction,  uniting  at  the  crossings  and 
forming  ganglia  there,  and  from  these  again  come 
smaller  branches  going  to  all  the  important  viscera  and 
controlling  their  functions  (Fig.  38,//). 

The  Plexuses. — Beginning  above  and  going  down- 
ward, the  principal  plexuses  are  (i)  the  carotid  and  (2) 
the  pharyngeal,  small  plexuses  with  their  ganglia  con- 
trolling the  throat  viscera  (Fig.  38,//  i  and  2);  (3)  the 
cardiac  (plexus  in  the  thorax)  with  its  ganglia,  control- 
ling the  action  of  the  heart  (Fig.  38,// 3);  (4)  in  the 
stomach  region  the  epigastric  or  solar  plexus  with  its 
ganglia,  controlling  the  action  of  the  stomach,  spleen, 
and  liver  (Fig.  38,^/4);  (5)  the  hypogastric  plexus  and 
its  ganglia,  controlling  the  functions  of  the  pelvic  vis- 
cera (Fig.  38,  //  5).  Into  the  cardiac  and  epigastric 
plexus  enter  the  branches  of  the  pneumogastric  nerve 
from  the  medulla,  and  play  an  important  part  in  the 
control  of  the  heart,  lungs,  and  stomach. 

Function. — The  function  of  this  system  is  obscure, 
but  certainly  largely  connected  with  the  processes  of  nu- 
trition, secretion,  etc.,  or  organic  functions.  Its  func- 
tion is  doubtless  also  reflex,  so  far  as  the  organs  to 
which  its  nerves  are  distributed  are  concerned,  but 
whether  by  its  own  fibers  or  by  means  of  fibers  derived 
from  the  axial  system  is  more  doubtful.  It  seems  to 
control  nutrition  and  secretion  by  controlling  the  blood 
supply;  and  this  is  done  by  means  of  certain  fibers — 
vasomotor  fibers — distributed  to  the  cajiillary  blood  ves- 
sels— vasomotor  nerves.      Cutting   the   vasomotor   nerves 


70 


PIIVSIOLOGV    AND    MORPHULOGV    OF    ANIMALS. 


seems  to  paralyze  the  smaller  blood  vessels,  which  then 
enlarge,  become  gorged  with  blood,  and  the  part  becomes 
finally  hot  and  inflamed.  Stimulation  of  these  nerves, 
on  the  contrary,  produces  contraction  of  these  blood 
vessels  and  coolness  and  paleness  of  the  part.  Blushing, 
on  the  one  hand,  and  the  paleness  of  terror,  on  the  other, 
are  supposed  to  arise  from  opposite  conditions  of  the 
vasomotor  nerves. 

Illustration  by  Telegraphy. — If  we  must  push  the  tele- 
graphic illustration  to  include  this  system  also,  then  it 
may  be  compared  to  a  municipal  government  control- 
ling local  affairs. 

SECTION    V. 

COMPARATIVE     PHYSIOLOGY     AND     MORPHOLOGY     OF     THE 
NERVOUS    SYSTEM. 

Introductory — Outline  of  the  Classification  of  Animals. 

About  to  enter  now  on  the  comparative  morphology 
and  physiology  of  the  nervous  system,  it  becomes  neces- 
sary to  have  in  mind  some  scheme  of  classification  of 
the  animal  kingdom.  A  true  classification  is  a  compen- 
dious expression  of  perfect  knowledge,  and  would  seem 
therefore  to  come  last  of  all.  But  some  provisional 
classification  is  a  necessary  condition  of  increase  of 
knowledge,  because  it  is  impossible  to  deal  scientifically 
with  animals  except  in  groups.  Therefore  our  plan 
will  be  to  give  a  simple  outline  of  such  a  classification 
and  to  verify  it  or  modify  it  as  we  proceed.  There  are 
a  great  variety  of  classifications  which  have  been  pro- 
posed, almost  as  many  as  the  proposers.  We  select  one 
which  is  probably  as  good  as  any,  and  has,  moreover, 
the  additional  advantage  of  comparative  simplicity,  for 
our  main  object  is  to  be  able  to  handle  the  material. 


NERVOUS   SYSTEM    OF    VERTEBRATES. 


71 


The  whole  animal  kingdom  may  be  primarily  divided 
into  seven  groups  called  subkittgdoms  or  departments  or 
phyla.  These  are  again  each  subdivided  into  classes,  and 
these  latter  into  orders,  families,  genera,  species,  etc.  In 
the  schedule  given  below  we  go  no  further  than  classes. 
Orders  will  be  referred  to  sometimes,  but  not  often. 
Even  some  classes  are  not  used. 


Metazoa. 

Proto- 
zoa. 

Articl'l.^ta. 

Mollusca. 

Radiata. 

Vertebrata. 

Arthropoda. 

Annelida. 

Echino- 
dermata. 

Coelen- 
terata. 

Proto- 
zoa. 

Mammals 

Birds 

Reptiles 

Amphibia 

Fishes 

Insects: 
Arachnids 
Myriapods 

Crustacea 

Annelids 

Cephalopods 
Gasteropods 
Acephala 
Brachiopods 

Echinoids 
Asteroids 
Crinoids 
Holothuri- 
oids 

Acalephae 
Polyps 

Infuso- 
ria 

Rhizo- 
pods 

These  groups  are  not  of  equal  value  or  significance, 
as  shown  above.  The  whole  animal  kingdom  may  be 
divided  into  two  prime  groups,  viz.,  protozoa,  or  sim- 
plest animals  consisting  of  one  cell  only,  and  metazoa,  or 
animals  consisting  of  an  aggregate  of  more  or  less 
differentiated  cells.  The  metazoa,  being  higher,  are 
more  differentiated,  and  therefore  are  divided  into  many 
great  departments.  Again,  I  have  linked  together  the 
echinoderms  and  coelenterates  under  the  name  radiata, 
as  having  a  common  radiated  plan  of  structure;  also  the 
arthropods  and  annelids,  or  segmented  worms,  under 
the  name  articulata,  as  having  a  common  jointed  or 
ringed  plan  of  structure.  We  shall  use  these  terms  in 
connection  with  the  general  laws  of  animal  structure.  I 
take  for  granted  that  the  student  already  has  some  gen- 
eral knowledge  of  zoology.  I  give  only  such  classifica- 
tions and  such  names  as  I  shall  use  in  the  comparison 
that  follows. 


n 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


COMPARATIVE    MORPHOLOGY     AND     PHYSIOLOGY     OF     THE 
VERTEBRATE    NERVOUS    SYSTEM. 

The  general  plan  of  the  nervous  system  is  so  pre- 
cisely the  same  in  all  vertebrates  that  only  the  most 
general  statements  are  necessary  in  regard  to  this.  In 
all  vertebrates,  but  in  no  other  animals,  we  have  both 
an  axial  and  a  ganglionic  system.  In  all  vertebrates 
the  axial  system  consists  of  a  continuous  tract  of  gray 
matter  inclosed  in  white  matter  lying  along  the  dorsal 
aspect  of  the  body,  enlarged  at  the  anterior  end  to  form 
a  brain,  and  giving  off  nerves  in  pairs  from  one  end  to 
the  other  (Fig.  14,  page  29).  In  different  vertebrates 
the  number  of  these  pairs  vary,  being  least  in  frogs  and 
toads,  where  there  are  only  eighteen  or  twenty,  and 
greatest  in  some  fishes,  as  the  eels,  where  they  may  be 
two  hundred  or  more.  Therefore  the  only  part  where 
the  differences  are  important  enough  to  arrest  our  at- 
tention in  this  rapid  sketch  is  the  brain. 

THE    BRAIN    OF    VERTEBRATES. 

In  running  down  the  vertebrate  scale  there  are  three 
important  changes  which  take  place  in  the  brain  :  \.  In 
size,  both  absolute  and  relative.  2.  In  relative  amount 
of  gray  matter  compared  with  white,  as  shown  by  the 
complexity  of  the  convolutions.  3.  In  the  relative  size 
of  the  cerebrum  as  compared  with  the  other  ganglia  of 
the  brain.  Perhaps  I  may  add  :  4.  In  the  relative  size  of 
the  frontal  lobe  compared  with  the  other  lobes  of  the 
cerebrum,  as  shown  by  the  position  of  the  fissure  of 
Rolando.  In  all  these  respects  the  brain  of  man  stands 
pre-eminent. 

I.  Size  {a)  Absolute. — The  brain  of  man  weighs  about 
three  pounds  (forty-eight  to  fifty  ounces).  The  heaviest 
which  have  been  weighed — viz.,  that  of  Cuvier,  the  great 


NERVOUS   SYSTEM    OF    VERTEBRATES. 


/J 


comparative  anatomist,  and  that  of  Turgenief,  the  great 
novelist — were  about  four  pounds.  It  varies  slightly 
in  different  races,  being  greater  in  the  superior  races, 
but  not  so  much  greater  as  might  have  been  expected. 
There  are  only  two  animals  that  have  larger  brains  than 
man,  viz.,  the  elephant,  whose  brain  is  about  eight 
pounds,  and  the  whale,  whose  brain  is  about  five  pounds. 
The  enormous  size  of  these  animals  is  sufificient  reason. 
[d)  Size  relative  to  the  Body  or  to  Rest  of  the  Nervous 
System. — This  is  far  more  significant  than  the  last.  The 
brain  of  the  highest  animal  of  like  size,  viz.,  the  gorilla 
is  only  about  one  third  that  of  man,  viz.,  fifteen 
ounces.  Below  this  there  is  a  constant  decrease  of  rela- 
tive size.  This  is  shown  in  the  following  table.  Of 
course  we  only  take  averages  of  these  various  classes. 


Classes. 

Fishes 

Reptiles 

Birds 

Mammals 

Man 


Brain 

to  nervous  system. 

; 

5 

3 

30 

-.1=   4 
:  5=  ^    . 
:  1=   5  times. 
:  1=   3  times. 
:  1=30  times. 

There  are  some  things  in  this  table  which  require  e.\- 
planation.  First,  it  is  seen  that  there  is  no  superiority 
in  reptiles  over  fishes  in  brain  to  body  weight,  but  there 
is  in  the  relation  to  the  rest  of  the  nervous  system. 
.•\gain,  it  is  seen  that  birds  are  apparently  superior  to 
mammals.  The  reason  of  this  is  that,  as  a  law,  small 
animals  have  larger  brains  proportionately  than  large 
animals.  Now  birds,  as  a  rule,  are  smaller  animals  than 
mammals.  Indeed,  some  of  the  smallest  birds,  such  as 
the  humming  bird  and  the  kinglet,  have  actually  larger 
brains  proportionately  than  man.  The  same  is  true  of 
the  smallest  mammals,  such  as  the  mouse.  But  there  are 
other  things   spoken  of  later,  viz.,  fineness  of  organiza- 


74 


PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


tion,  which  determine  intellect  quite  as  much  as  or  more 
than  size.     Perhaps  it  might  be  well  to  say  here,  for  the 

comfort  of  those  who 
wear  small-sized  hats, 
that  brain  power  does 
not  depend  on  size 
alone,  any  more  than 
bodily  strength  de- 
pends on  weight  alone. 
In  both  cases  it  is  a 
product  of  two  factors, 
viz.,  size  and  fineness 
of  organization,  and 
the  latter  is  the  more 
important  factor. 

Brains  of  Extinct 
Species.  —  It  is  a  cu- 
rious and  significant 
fact  that  in  each  of 
these  classes  extinct 
species  are  remarka- 
ble for  the  smallness 
of  their  brains.  There 
has  been  a  gradual  in- 

FlG.   39.— ^,  outline  of  the  skull  and  brain  Crease    in    the    size    of 

cavityoflchthyomis  victor  I  after  Marsh)  j^g    brains   of    animals 
seen   from   above,     (tive  sixths  natural 

size.)     5,  outline  of  the  skull  and  brain  in  each  of  these  claSSeS 

cavity  of  Sterna  cantiaca  ( after  Gmelin),  -             »i,    "      fi      t    •    «. 

same  view.    (Natural  size. )    <?/,  olfactory  trom   tneir   nrst   iniro- 

lobes;  c,  cerebral  hemispheres;  op,  optic  (Juction  until  nOW.     To 
lobes;  cb,  cerebellum. 

give  one  example,  the 
extinct  Cretaceous  bird  Ichthyornis  was  about  the  size 
of  a  tern,  but  its  brain  was  hardly  one  quarter  as  large 

(Fig-  39)- 

2.  Relative  Amount  of  Gray  Matter.— The  gray 
matter  is  \.\\&  generator,  the  white  fibers  only  transmitters  of 


NERVOUS    SYSTEM    OF    VERTEBRATES. 


75 


Fig.  40. — Brain  of  Ornitho- 
rhynchus.  (Natural  size. ) 
( After  Parker. ) 


nerve  force.  The  former,  therefore,  is  the  higher.  The 
organization  of  the  brain  is  tested  by  the  relative  amount 
of  gray  matter.  Comparing  again 
to  electricity,  the  electro-motive 
force  varies  as  the  gray  matter. 
Further,  this  relative  amount, 
other  things  being  equal,  is  ex- 
pressed by  the  number  and  depth 
of  the  convolutions.  Now,  of  all 
animals  the  number  and  depth  of 
the  convolutions  of  the  cerebrum 
is  by  far  greatest  in  man.  In 
higher  mammals,  especially  an- 
thropoid apes,  the  convolutions 
are  well  marked,  but  they  become 
less  and  less  so  as  we  go  down 

the  mammalian  scale,  until  they  entirely  disappear  before 
we  reach  the  lowest  mammals  (Fig.  40).  Therefore  all 
below  mammals — i.e.,  all  birds,  reptiles,  and  fishes — have 
smooth  brains. 

There  is,  however,  a  modifying  cause  here  which 
must  not  be  neglected  ;  it  is,  that  the  brains  of  small 
animals  tend  to  smoothness  irrespective  of  deficiency  of 
gray  matter.  The  reason  is  that  bulk  varies  as  the  cube, 
while  surface  only  as  the  square  of  the  diameter,  and 
therefore  a  small  sphere  has  proportionately  a  greater 
surface  than  a  large  sphere.  It  follows  from  this  that 
the  brain  of  a  small  animal,  though  smooth,  may  have 
as  much  gray  matter  proportionately  as  the  highly  con- 
voluted brain  of  a  large  animal.  Thus  all  small  mam- 
mals have  smooth  brains,  while  large  mammals  have  all 
convoluted  brains.  The  brain  of  an  elephant,  or  even 
of  a  whale,  is  wonderfully  convoluted,  almost  as  much 
so  as  that  of  man. 

As  we  go  back  in  the  embryonic  series  the  brains  of 


76      PHYSIOLOC^A'    AND    MORPHOLOGY    OF    ANIMALS. 

mammals  and  of  man  become  less  and  less  convoluted, 
until  they  become  entirely  smooth.  The  same  is  true 
as  we  go  back  in  the  evolution  series.  Extinct  mam- 
mals have  less  and  less  convoluted  brains  as  we  go  back 
in  time.     All  the  earliest  mammals  had  smooth  brains. 

3.  Relative  Size  of  Cerebrum. — The  cerebrum  is 
confessedly  the  highest  part  of  the  brain.  The  relative 
size  of  this  is  the  best  of  all  tests  of  position  in  the  scale 
of  organization.  Observe,  then,  that  there  are  four 
lobes  used  in  this  comparison,  viz.,  the  cerebellum,  the 
optic  lobes,  the  cerebrum,  and  the  olfactory  lobes  ;  for 
this  last  is  an  important  lobe  in  all  lower  vertebrates. 

Now  in  man,  as  alVeady  seen,  the  cerebrum,  growing 
out  from  the  thalamus,  spreads  forward,  covering  en- 
tirely the  olfactory  lobes,  and  backward,  covering  first 


Fig.  41. — Mammal  brain.     A,  top  view  ;  B,  side  view  of  the  brain  of  a  cat. 


the  optic  lobes,  then  the  cerebellum,  until  looking  down 
upon  the  brain  we  see  nothing  else.  It  covers  and 
dominates  all.  In  monkeys,  by  a  less  backward  prolon- 
gation, the  cerebellum  begins  to  peep  out  behind  In 
the  average  mammals,  such  as  the  lion  or  the  dog,  the 
olfactory  lobes  are  exposed  in  front,  and  nearly  the 
whole  of  the  cerebellum  is  uncovered  behind  (Fig.  41). 
Still  lower  among  mammals  the  cerebellum  is  wholly 
uncovered  and  the  ojnic  lobes  begin  to  appear,  and  all 
the  four  lobes  are  seen  in  a  series  (Fig.  42).  The  brains 
of  extinct  mammals  are  all  of  this  low  type. 


NERVOUS    SYSTEM    OF    VERTEBRATES. 


77 


Owens  Classification  of  Mammals. — Professor  Owen 
classified  mammals  by  this  test  into  four  groups :  i. 
Archencephala,  or  ruling  brain.  In 
this  subclass  he  placed  man  alone. 
2.  Gyrencepkala,  or  convoluted 
brains.  In  this  he  placed  all  the 
larger  mammals.  3.  Lisscncephala, 
or  smooth  brains.  In  this  he 
placed  all  the  rodents  and  some 
other  small  mammals.  4.  Lyen- 
^^//d;/a,  or  separated  brains.  The 
lowest  mammals,  such  as  insecti- 
vores  and  marsupials,  he  placed 
in  this.  This  attempt  is  interest- 
ing, but  the  classification  can  not 
be  regarded  as  natural,  for  the 
earliest  animals  of  all  these  sub- 
classes, except  one — man — have 
smooth  brains. 

In  the  average  bird  (Fig.  43)  not  only  is  the  olfac- 
tory lobe  wholly  uncovered  in  front  and  the  cerebellum 
behind,  but  also  the  optic  lobes  are  at  least  half  un- 
covered. 


t _, 

Fig.  42. — Brain  of  Das)rurus 
ursinus,  showing  expo- 
sure of  the  optic  lobes,  ol. 
(From  Owen.) 


Fig.  43. — Bird  brain  :  A,  side  view  ;  B,  top  view.* 

In   the  reptile    (Fig.  44)    all  the   lobes  are   fully  ex- 
posed, and  the  brain  becomes  a  succession  of  lobes  in  a 


*  In  all   these  fii^ures'to  54:   w,  medulla;  </',  cerebellum;  01, 
optic  lobes  ;  cr,  cerebrum  ;  of,  olfactory  lobes, 


78      PHYSIOLOGY   AND    MORPHOLOGY    OF    ANIMALS. 

linear  series,  but  the  cerebrum  still  maintains  its  pre- 
eminence as  the  larjjest  of  the  series. 


Fig.  44. — Reptile  brain  :  A,  side  view  ;  B,  top  view. 

Finally,  in  fishes  (Fig.  45)  this  pre-eminence  of  the 
cerebrum  is  lost  and  the  optic  lobes  are  the  largest.  In 
the  very  lowest  fishes,  such  as  the  lampreys  [Fetromy- 
zon),  there  is  scarcely  any  enlargement  at  the  anterior 
end,  and  in  the  lancelet  {AmpJiioxus,  Fig.  46)  the  cord 
is  continued  into  the  head  with  no  perceptible  enlarge- 
ment at  all. 


Fig.  45. — Fish  brain  :  A,  side  view;  B,  top  view. 

In  all  these  gradual  changes  by  which  the  brain  is  re- 
duced finally  to  a  linear  series  of  swellings,  it  is  remark- 
able to  see  how  persistent  are  two  little  organs,  the 
functions  of  which  are  still  doubtful,  one  below  and  one 

V         hi 


Fig.  46. — Amphioxus  :  spc^  spinal  cord. 

above   the   thalamus,  viz.,  the  pituitary   gland  and  the 
pineal  gland.     These  are  seen  in  Figs.  48-53.      The  use 


NERVOUS    SYSTEM    OF    VERTEBRATES. 


79 


of  the  former,  if  any,  is  unknown.  The  latter  seems  to 
be  a  useless  remnant  of  a  once  useful  eye  in  the  top  of 
the  head.  In  some  lizards  it 
still  retains  the  structure  of  an 
eye  (Fig.  47). 

Embryonic  compared  linth  the 
Taxonomic  Series. — It  is  a  most 
significant  fact  that  the  brain 
of  the  embryo  of  man  passes 
through  all  these  stages:  (i) 
The  earliest  condition  of  the 
human  brain  is  seen  in  Fig.  48. 
This  can  be  compared  with  the 
brain  of  only  the  very  lowest 
fishes.  It  may  therefore  be 
called  the  subfish  stage.  It  does 
not  yet  contain  a  cerebrum. 
(2)  Then  the  cerebrum  grows 
out  of  the  thalamus,  but  is  yet  inferior  in  size  to  the 
optic  lobes  (Fig.  49).  This  may  be  called  the  average 
fish  stage.  (3)  Then  the  cerebrum  grows  until  it  is  the 
largest  of  the  series,  but  covers  nothing  as  yet  (Fig. 
50).  This  may  be  called  the  reptile  stage.  (4)  Then  it 
begins  to  cover  the  optic  lobes  (Fig.  51).     This  corre- 


FiG.  47. — Parietal  eye  of  Hat- 
teria  (after  Spencer) :  /,  lens  ; 
V,  vitreous  humor  ;  r,  ret- 
ina ;  0,  optic  nerve. 


Fig.  48. — Sub-fish  stage  :  ///,  thalamus ;  ol,  optic  lobe ;  w,  medulla. 


sponds  to  the  bird  stage.     (5)  Then  it  covers  the  whole 
of  the  optic  lobe  and  encroaches  on  the  olfactory  lobe 
7 


8o      PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 

in    front    and   the   cerebellum   behind    (Fig.    52).     This 
corresponds  to   the  condition   of  the  average  mammal, 


Fig.  49.— Fish  stage. 

and  may   be  called   the  mammal  stage.     (6)    Finally,    it 
grows  forward,  wholly  covering  the  olfactory  lobe,  and 


Fig.  50. — Reptile  stage. 

backward,  wholly  covering  the  cerebellum,  and  we  have 
the  human  stage  (Fig.  53). 


Fig.  51.— Bird  stage. 

Meanwhile  several  other  changes  are  in  progress  :  (i) 
In  proportion  as  the  head  is  raised  nearer  and  nearer  to 
a  vertical  position,  the  base  of  the  brain,  which  was  at 


NERVOUS    SYSTEM    OF    VERTEBRATES.  gl 

first  in  direct  line  with  the  cord,   begins  to  bend  more 
and  more  until  it  is  at  right  angles  in  man  ;  (2)  the  cere- 


FlG.  52. — Mammalian  stage. 

helium  has  been  increasing  in  relative  size;  and  (3)  the 
convolutions  of  both  the  cerebellum  and  the  cerebrum 


Fig.  ^2- — Human  stage. 

have  been  becoming  more  and  more  complex.     All  these 
changes  are  combined  and  re[)resented  in  Fig.  54. 


82      PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 

4-  Relative  Size  of  Frontal  Lobe.— The  highest 
functions  of  the  intellect  are  connected  with  the  frontal 
lobe  of  the  cerebrum.  This  is  marked  off  by  the  fis- 
sure of  Rolando.  In  man  the  portion  anterior  to  this 
fissure  is   large.     In   monkeys   the  fissure  is  plain,   but 


Man 


Fig.  54. — Diagram  showing  all  the  stages  in  one. 

the  anterior  lobe  is  much  smaller,  and  increasingly  so 
as  we  go  down  the  scale  of  monkeys.  The  fissure  is 
not  certainly  discernible  at  all  below  monkeys. 


CEPHALIZATION. 

This  may  be  defined  as  headward  development.  Intro- 
duced by  Dana  to  express  the  gradual  transfer  of  func- 
tions headward  by  the  modification  of  homologous  organs 
as  we  go  up  the  scale  of  Crustacea  (see  page  268),  we 
here  use  it  in  a  similar  but  a  wider  sense  as  the  gradual 
increasinfT  dominance  of  higher  over  lower  functions  in 


NERVOUS   SVSTExM    OF    VERTEBRATES.  83 

the  process  of  evolution  and  usually  accompanied  with 
transfer  headward. 

In- the  process  of  development,  whether  in  the  evolu- 
tion series,  or  in  the  taxonomic  series,  or  in  the  embry- 
onic series,  we  observe  the  same  order.  Organisms  are 
at  first  unmodified  cell-aggregates.  From  such  aggre- 
gates tissues  performing  different  functions  are  differ- 
entiated. From  this  time  onward  cephalization  begins. 
Among  the  tissues  there  is  a  gradually  increasing  domi- 
nance of  the  highest,  the  controlling  tissue,  viz.,  the 
nervous  tissue.  Then  in  the  nervous  tissue  a  gradually  in- 
creasing dominance  of  the  highest  part,  viz.,  the  braiji. 
Then  in  the  brain  a  gradually  increasing  dominance  of 
the  highest  ganglion,  viz.,  the  cerebrum.  Then  /';/  the 
cerebrum  a  gradually  increasing  dominance  of  the  high- 
est substance,  the  surface  gray  matter,  as  shown  by  the 
complexity  of  the  convolutions.  And,  lastly,  among  the 
convolutions  a  gradually  increasing  dominance  of  the  high- 
est, viz.,  those  in  the  frontal  lobe,  as  shown  by  the 
position  of  the  fissure  of  Rolando.  In  all  there  is  an 
increasing  dominance  of  the  higher  over  the  lower,  and 
of  the  highest  over  all.  This  is  everywhere  the  law  of 
evolution. 

Shall  it  stop  here  ?  Shall  it  not  be  carried  forward 
on  a  higher  plane  by  the  conscious  effort  of  man  ?  Is 
not  all  civilization,  all  culture,  all  education  a  voluntary 
process  of  cephalization  ?  Here,  also,  there  must  pre- 
vail the  same  law  of  progressive  domination  of  the 
higher  over  the  lower,  of  the  distinctively  human  over 
the  animal,  of  mind  over  body;  and,  in  the  mind,  of  the 
higher  faculties  over  the  lower,  the  reflective  over  the 
perceptive,  and  of  the  moral  character  over  all.  In  all 
your  culture  be  sure  that  you  strive  to  follow  this  law 
of  evolution. 


^4      PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

SECTION    VI. 

Nervous  System  of  Invertebrates. 

The  departments  below  the  vertebiates  we  group 
together  under  the  general  term  invertebrates,  not  be- 
cause they  are  more  nearly  related  to  one  another  than 
they  are  to  vertebrates,  for  this  is  not  true,  but  because 
we  must  treat  them  far  more  cursorily. 

I.  Articulata.  Arthropods  and  Annelids. — The  plan 
of  structure  of  these  is  widely  different  from  that  of 
vertebrates.  In  these  (i)  the  skeleton  is  on  the  out- 
side, the  muscles  acting  from  within  to  produce  motion 


Fig.  55. — Diag;ram  of  section  across  an  arthropod  :  b,  blood  system  ;  «,  nerv- 
ous system  ;  i,  intestines. 

and  locomotion  ;  instead  of  being  within,  and  muscles 
working  on  the  outside  for  the  same  purpose,  as  in  the 
vertebrates.  (2)  Of  all  departments  these  animals  are 
the  most  distinctly  and  most  numerously  jointed,  and 
therefore  they  are  called  articulata.  (3)  The  skeleton 
being  on  the  outside,  it  forms  a  hollow  tube  or  cavity 
inclosing  all  the  viscera  and  organs  of  the  body  (Fig.  55), 


NERVOUS    SYSTEM    OF    INVERTEBRATES. 


,.  vert 
.b 


while  in  vertebrates  there  is  a  separate  cavity  inclos- 
ing and  protecting  the  nervous  centers  (Fig.  56).  (4) 
In  the  vertebrates,  as  we  have 
seen,  the  nerve  center  con- 
sists of  a  continuous  tract  of 
gray  matter  lying  along  the 
<^/.y<// aspect  of  the  body,  above 
the  visceral  canal.  In  the  ar- 
ticulata,  on  the  contrary,  the 
nerve  center  is  a  continuous 
chain  lying  along  the  ventral 
aspect  of  the  body,  below  the 
visceral  canal.  (5)  In  the  case 
of  vertebrates,  as  already  seen, 
the  nerve  system  is  subdivided 
into  two  subsystems,  the  axial 
and  the  ganglionic.  In  the  case 
of  the  articulata  there  is  but 
one  system,  which  probably  per- 
forms the  functions  of  both,  these  subsystems  not  having 
yet  been  differentiated. 

It  is  difficult,  indeed  impossible,  to  conceive  how  the 
vertebrate  nervous  system  could  have  been  evolved  out 
of  that  of  the  articulates.  If  vertebrates  came  as  a 
branch  from  the  articulates,  as  many  think,  they  must 
have  come  off  so  low  down  that  the  distinctive  plan  of 
neither  was  yet  declared. 

In  treating  of  the  plan  of  the  nervous  system  we 
shall  take  all  the  articulata  together,  as  the  plan  is  the 
same  in  all,  although  most  distinct  in  the  arthropods. 

General  Plan  of  Articulate  Nervous  System. — To  bring 
this  out  clearly  it  is  best  to  take  an  example  from  about 
the  middle  of  the  series — as,  for  instance,  a  leech,  or  a 
crayfish.  In  Fig.  57  we  give  a  side  view  of  the  nervous 
system  of  one  of  the  lower  Crustacea,  and  in  Figs.  58  and 


Fig.  56. — Cross  section  through 
a  fish  :  vs^  visceral  system  ; 
vert,  vertebra  ;  ^,  bleed  sys- 
tem ;  ns,  nervous  system. 


86      PHYSIOLOGY    AND    MORPHOLOGY    OF    ANL\L\LS. 

59  a  back  view  of  that  of  a  leech  and  of  a  crayfish.  It 
may  be  most  simply  described  as  consisting  of  a  chain 
of  ganglia  strung  along  the  whole  ventral  aspect  of  the 
body  and  below  the  alimentary  canal,  one  to  each  joint, 
connected  by  a  single  or  double  thread,  and  the  most  ante- 
rior one — the  a'sop/iagea/ gcingliou — being  connected  with 
a  large  ganglion  in  the  head,  above  the  gullet — cephalic 
ganglion — by  two  threads,  one  on  each  side  of  the  gul- 

bs  a/s  ns 


Fig.  57. — Diagram  of  a  crustacean  (  Vibilia\  :  ds,  blood  system  ;  a/s,  alimen- 
tary system  ;  ns,  nervous  system. 

let,  to  form  the  (esophageal  collar.  It  is  a  remarkable  fact 
that  an  oesophageal  collar  is  found  in  nearly  all  inverte- 
brates. 

Functions  of  the  Several  Ganglia. — The  cephalic  gan- 
glion seems  to  preside  over  the  higher  senses,  viz.,  the 
eyes  and  the  antenncB.  It  is  also  the  seat  of  conscious- 
ness and  volition  and  of  whatever  instinct  or  intelli- 
gence the  creature  is  possessed  of,  and,  through  its  con- 
nections with  other  ganglia,  it  dominates  the  whole  body. 
In  a  word,  it  corresponds  in   function  to  the  cerebrum 


iNERVOUS   SYSTEM    OF    INVERTEBRATES. 


87 


of  vertebrates.  The  oesophageal  ganglion  presides  over 
the  gathering  and  mastication  of  food,  and,  judging 
from  experiments  on  crustaceans,  it  seems  also  to  co- 
ordinate the  motions  of  the  _ 

W 


whole  body.  It  may  be  said 
to  correspond  in  function 
to  the  cerebellum  of  verte- 


< 


Fig.  58. — Diagram  nf  nervous  sys- 
tem of  a  leech,  seen  from  above. 


Fig.  59. — Nervous  system  of  a  cray- 
fish, seen  from  above. 


brates.  The  other  ganglia  preside  each  over  its  own 
body-segment  and  corresponding  limbs  automatically, 
but  all  under  the  conscio-voluntary  control  of  the  oeso- 
phageal collar,  and  especially  of  the  cephalic  ganglion. 

Modifications  going  down  and  up  the  Scale. — 
Taking  the  above  as  the  type,  as  we  go  down  the  scale 
of  articulates,  the  cephalic  ganglion  becomes  smaller  in 
comparison  with  the  others,  and  loses  more  and  more  its 
dominance  over  them.  The  other  ganglia  become 
more  and  more  independent,  and  the  movements  more 
and  more  automatic  or  reflex,  until  finally,  in  the  lowest 


88      PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 

worms,   the   different    segments   become    almost   wholly 
independent,  and  indeed  almost  like  separate  animals. 

In  going  ///  the  scale,  on  the  contrary,  we  find  oppo- 
site changes  of  two  kinds,  viz.,  centralization  and  cephali- 
zation.  Centralization  reaches  its  highest  degree  in  crabs 
and  spiders,  where  all  the  ganglia  except  the  cephalic 
are  consolidated  into  one  in  the  center  of  the  cephalo- 
thorax,  and  therefore  in  the  vicinity  of  the  stomach  and 


..N..  \.-'^i 


ir 


.,-;^-r:-^ 


U...'.V'.V.'Jii.  '•■- -..-'l3'"""X'---' 

~ '-/V.;r.v.;;.-<r--'-''^V 

4-'' '  '"'-^ 

Fig.  6o. — Xen-ous  system  of  a  crab. 


Fig.  6i. — Diarram  of  nerv- 
ous system  of  a  beetle. 


locomotive  organs  (Fig.  6o).  The  cephalization  reaches 
its  highest  degree  in  the  high  insects,  as  bees,  flies,  beetles, 
etc.  (Fig.  6i). 

Embryonic  History  of  the  Nervous  System. — The  same 
changes  are  gone  through  in  the  embryonic  history  of 
one  of  the  higher  insects,  such  as  the  butterfly.  In  the 
caterpillar  all  the  ganglia  are  separate,  one  to  each  seg- 
ment. In  the  chrysalis  the  cephalic  ganglion  increases 
in  size,  and  three  or  four  of  the  anterior  body  ganglia 
are  drawing  closer  together.  In  the  butterfly  the  cephalic 
ganglion  is  still  larger,  and  four  of  the  ventral  ganglia, 
together  with  the  oesophageal,  have  united  into  a  great 
thoracic  ganglion  to  control  the  powerful  muscles  moving 


NERVOUS   SYSTEM    OF    INVERTEBRATES 


89 


the  wings  and  legs,  as  well  as  to  preside  over  the  stomach, 
etc.  (Fig.  62).    It  is  well  to  observe  that  the  consolidation 


-i^ 


Fig.  62. — Diagram  showing  the  embryonic  development  of   a  lepidopter  : 
a,  caterpillar  ;  d,  chrj'salis  ;  c,  the  perfect  butterfly. 


of  the  segments  of  the  skeleton  follows  the  same  course 
— goes  hand  in  hand  with  that  of  the  nervous  system. 

2.  Mollusca.  Comparison  ivith  Other  Departments. — 
The  whole  plan  of  structure  of  these  is  again  different. 
The  vertebrates  and  arthropods  both  have  a  true  loco- 
motive skeleton,  the  one  interior,  the  other  exterior.  In 
both,  also,  but  especially  the  latter,  the  skeleton  con- 
sists of  segments  repeated  in  a  linear  series.  The  mol- 
lusks  have  no  locomotive  skeleton,  but  only  a  pro- 
tective shell.      The  mollusks  also   have  no   segmented 


90 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


structure,  no  repetition  of  similar 
parts  in  a  linear  series.  Now  it  is 
probable  that  the  nervous  system 
controls  the  general  structure  of 
the  body.  Thus  while  the  nervous 
system  of  vertebrates  is  a  continu- 
ous axis,  and  that  of  arthropods  and 
worms  a  string  of  ganglia  running 
through  the  body,  in  moUusks  there 
is  no  axial  arrangement  as  in  verte- 
brates, nor  a  linear  series  of  gan- 
glion as  in  arthropods. 

General  Plan  of  the  Ne7-vous  Sys- 
tem.— In  these  animals  the  nervous 
system  consists  (i)  of  an  eesophageal 
collar,  and  (2)  of  ganglia  irregular- 
ly placed  wherever  important  func- 
tions, either  nutritive  or  locomo- 
tive, are  situated. 

Examples. —  i.   In  bivalves  (acephala),  such  as  clams, 
for  instance,  we  have  (Fig.  63)  :  (i)  The  oesophageal  col- 
lar, which  presides  over  the  mouth  and  head  functions 
(e.  g.,    gathering   of    food    and 
whatever    beginnings    of  intelli- 
gence the  creature  may  possess), 
and  also  has  general  presidence 
over  conscious  voluntary  move- 
ments, and  therefore  over  other 
ganglia;  (2)  a  large  visceral  gan- 
glion in  the  region  of  nutritive 
and  respiratory  organs,  to  pre- 
side over  these ;  and  (3)  a  loco- 
^      ^      ^,  ,        ,     motive  or  foot  ganglion  to  con- 

FlG.  64. — Nervous  system  of  .  ,    .     . 

an  oyster :  c,  cephalic  gan-     trol  locomotion.    This  is  all.    In 

elion ;  v,  the  visceral  gan-  ,        ,  i  *.    „  • 

llion.  the  oyster  the  nervous  system  is 


Fig.  63. — Nervous  system 
of  a  clam  :  eg,  cephalic 
ganglion  ;  pg^  pedal 
ganglion  ;  vg^  visceral 
ganglion. 


NERVOUS   SYSTEM    OF    INVERTEBRATES. 


9' 


still    simpler,  since    it    lacks    the    locomotive    ganglion 
(Fig.  64). 

2.  In  gastropods,  as  the  snails  (Fig.  65),  we  find, 
(1)  as  usual,  the  cephalic  ganglia  and  oesophageal  col- 
lar in  which  are  lodged  all  the  higher  functions — con- 
sciousness and  volition,  control  of 
voluntary  movements,  etc. ;  (2)  in 
the  foot  or  crawling  disk  a  gan- 
glion to  preside  over  locomotion  ; 
(3)  visceral  ganglia  in  the  visceral 
region,  appropriately  placed.  All 
are,  of  course,  connected  with  the 
cephalic  ganglion  and  dominated 
by  it.  In  the  figure,  however,  only 
the  oesophageal  collar  is  repre- 
sented. 

3.  In  cephalopods  (squids,  cut- 
tlefish, etc.)  (Fig.  66)  we  have  (i) 
a  large  ganglion  in  the  head  com- 
pletely and  closely  encircling  the 
gullet,  and  forming  a  very  per- 
fect close-fitting  oesophageal  col- 
lar. This  is  doubtless  a  combina- 
tion of  cephalic  and  oesophageal 
ganglia.  It  controls  the  move- 
ments of  the  jaws  and  arms,  and 
it  presides  over  the  higher  senses  of  sight  and  hear- 
ing, which  are  well  developed  in  these  animals.  It  is 
the  seat  of  conscious  voluntary  motion  and  of  what- 
ever higher  faculties  the  creature  may  possess.  (2)  Be- 
sides this  there  are  a  pair  of  locomotive  ganglia  in  the 
muscular  mantle  to  control  its  contraction.  (3)  A  large 
visceral  ganglion  presiding  over  nutrition  and  respira- 
tion. All  of  these  are,  of  course,  connected  with  the 
head  franelion. 


Fig.  65. — Nervous  system 
of  a  gastropod  :  c,  ce- 
phalic ganglion  ;  e,  oeso- 
phageal ganglion. 


92 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


3.  Radiata. — We  include  in  these  the  echinoderms 
and  the  coelenterates,  as  having  the  same  general  plan 
of  nervous  system.  Here  again  we  have  the  whole  plan 
of  structure  of  the  animal  different.  We  have  again 
^segments  of  the  body,  not,  however,  repeated  in  a  linear 


Fig.  66. — Diagram  of  nervous  system  of  a  squid  :   eg,  cephalic  ganglion ; 
oeff,  oesophageal  ganglion  ;  ?ng,  mantle  ganglion. 


series,  but  in  a  circle  about  the  stomach,  like  the  seg- 
ments of  an  orange  about  the  pit  or  the  spokes  of  a 
wheel  about  the  hub.  Therefore  we  find  that  the  nerv- 
ous system  follows  the  same  plan  (Fig.  67). 

General  Plan. — Take  a  starfish  as  an  example.     There 
is  a  ganglion  at  the  base  of  each  arm,  and  therefore  five 

surrounding  the  mouth,  con- 
nected into  a  perfect  oeso- 
phageal collar.  From  these 
ganglia  there  go  nerves,  one 
to  each  arm,  giving  branches 
to  the  arm  and  terminating  at 
the  extremity  in  an  eye  spot. 
In  the  medusae  we  have  the 
same  perfect  radiated  struc- 
ture, but  the  nervous  system 
has  as  yet  no  general  center. 
From  the  several  eye  spots  on 
the  margin  of  the  disk  there  go  nerve  cords  a  little  way 
toward  the  center  of  radiation,  but  they  do  not  meet 
one  another  in   a  common  center.     It  is  difficult  to  see 


Fig.  67. — Diagram  of  the  nerv 
ous  system  of  a  starfish. 


NERVOUS    SYSTEM    OF    INVERTEBRATES. 


93 


how  such  an  animal   can   have  a  common  consciousness 
(Fig.  68). 

4.  Protozoa. — As  these  consist  of  a  single  cell  and 
not  even  a  cell  aggregate,  and  therefore  can  have  no 
tissue  of  any  kind,  they 
have  no  nervous  system. 
Yet  there  is  a  general  sen- 
sibility or  response  to 
stimulus  even  in  these  lit- 
tle spherules  of  proto- 
plasm, x^ll  functions  ai'e 
performed,  though  imper- 
fectly, by  these  living 
drops  of  jelly.  This  is 
the  starting  point  from 
which  by  evolution  have 
been  differentiated  all  the  tissues  and  organs  and  func- 
tions found  in  higher  departments. 


Fig.  68. — Diagram  of  a  medusa  :  n  n, 
nen'e  ;  m,  mouth  ;  st,  stomach  ;  rt, 
radiating  tubes. 


CHAPTER    II. 

SENSE    ORGANS. 

SECTION    I. 
Introductory. 

A  NERVOUS  system,  as  seen,  consists  essentially  of  a 
center  and  two  kinds  of  fibers,  one  carrying  impres- 
sions from  the  external  world  to  the  center  and  pro- 
ducing changes  in  consciousness,  which  we  call  sensation, 
the  other  carrying  impulses  back  from  the  center  to  the 
external  world  and  producing  changes  of  phenomena 
there.  But  to  make  these  exchanges  more  efficient  there 
must  be  special  receptive  organs  of  sensation  (these  are 
the  sense  organs)  and  special  executive  organs  of  the  will 
(these  are  the  muscles).  We  have  now  to  do  only  with 
the  sense  organs.     We  shall  speak  of  the  muscles  later. 

Relation  of  Special  Sense  to  General  Sensibil- 
ity.— The  sensory  fibers  of  the  conscio-voluntary  sys- 
tem terminate  peripherally  in  external  sensitive  surfaces, 
and  in  the  lowest  animals  everywhere  alike — i.  e.,  in  the 
skin.  This  gives  only  the  existence  of  an  external  world, 
but  not  \l% properties.  This  is  the  case  in  all  the  lowest 
animals,  and  nothing  more.  But  as  we  go  up  the  animal 
scale  certain  fibers  are  specialized  to  give  knowledge  of 
certain  properties,  and  certain  other  fibers  to  give  knowl- 
edge of  other  properties.  Thus,  for  example,  the  fibers 
of  the  first  pair  of  cranial  nerves  are  specialized  to  give 
us  cognizance  of  odors,  and  nothing  else,  and  any  kind  of 
stimulation  of  this  nerve  will  produce  a  perception  of 
94 


SENSE    ORGANS. 


95 


<^Wc\ 


odor.  The  fibers  of  the  second  pair  of  cranial  nerves  are 
specialized  in  such  wise  as  to  respond  to  impressions  of 
light,  and  nothing  else,  and  any  stimulation  of  this  nerve, 
whether  by  prick- 
ing or  by  electric- 
ity or  otherwise, 
gives  rise  to  a 
flash  of  light.  The 
eighth  pair  is  so 
specialized  as  to 
respond  to  vibra- 
tions of  the  air, 
and  any  stimula- 
tion of  this  nerve 
gives  rise  to  a  sen- 
sation of  sound. 
Similarlytheninth 
pair  is  so  special- 
ized as  to  produce 
the  peculiar  sen- 
sation which  we 
call  taste,  and 
nothing  else,  and 
hence  stimulation 
of  this  nerve  in 
any  way  will  pro- 
duce this  peculiar 
sensation.  This  is 
only  one  example 
of  the  universal  law  of  differentiation,  but  so  admirable 
a  one  that  we  stop  a  moment  to  dwell  upon  it. 

Commencing  with  the  lowest  anim.als — protozoa — or 
with  the  earliest  embryonic  condition  of  the  higher,  we 
have  first  a  single  cell,  and  then  an  unmodified  cell  ag- 
gregate.     As  we  pass  upward  three  fundamental  systems 


Uiz  niociiyiecZ 
Cell u tar    Structure. 

Fig.  69. 


96 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


separate,  having  each  a  characteristic  fundamental  func- 
tion, viz.,  the  epithelial  or  nutritive  system,  the  blood 
system,  and  the  nervous  system  (Fig.  69).  The  func- 
tion of  the  first  is  the  exchange  of  matter  with  the  ex- 
ternal world — foreign  commerce;  of  the  second,  ex- 
change of  matter  between  different  parts  of  the  body 
— internal  carrying  trade ;  of  the  third,  the  exchange  of 
intelligence,  both  foreign  and  interior.  Putting  aside 
the  other  two  (which,  however,  are  also  differentiated), 
we  take  only  the  nervous  system.  In  vertebrates  this 
again  is  differentiated  into  three  subsystems,  viz.,  the 
conscio-voluntary,  the  reflex,  and  the  ganglionic.  Put- 
ting aside  again  all  but  the  conscio-voluntary,  and  tak- 
ing here  only  the  sensory  fibers,  these  are  again  differ- 
entiated into  five  kinds,  viz.,  the  five  special  senses. 
Even  these  are  probably  further  differentiated — i.  e.,  dif- 
ferent kinds  of  colors,  tones,  feelings,  etc. — and  per- 
ceived by  different  fibers  differently  specialized. 

Now  as  different  as  these  kinds  of  sensation  are  from 
one  another,  so  different  that  they  can  not  be  conceived 
the  one  in  terms  of  another,  yet  they  are  all  probably 
but  modifications  of  one  another  and  all  refinements  of 
the  lowest — viz.,  feeling.  It  will  be  interesting,  then,  to 
trace  the  gradations  between  them  in  several  respects. 

I.  Vibrations. — Coarse  vibrations  are  perceived  by 
the  nerves  of  common  sensation.  We  call  them  jars  or 
tremors.  If  these  increase  in  number  and  decrease  in 
size  until  they  lose  their  separateness — i.  e.,  until  there 
are  sixteen  in  a  second,  then  they  appear  in  conscious- 
ness in  another  and  entirely  different  form — as  sound — 
and  we  have  a  special  nerve  adapted  to  perceive  these 
more  rapid  vibrations.  As  the  vibrations  become  more 
and  more  rapid  they  are  perceived  as  higher  and  higher 
musical  pitch,  until  they  reach  some  thirty  thousand  to 
forty  thousand  in  a  second,  which  is  the  acutest  sound 


SENSE   ORGANS. 


97 


we  can  hear.  Beyond  this  the  vibrations  exist,  but  we 
have  no  nerve  specialized  to  perceive  them.  After  a 
long  interval  vibrations  again  appear  in  consciousness, 
but  in  a  new  form — as  light.  This  only  takes  place  when 
they  reach  a  rapidity  which  can  be  taken  on  only  by  the 
ethereal  medium,  viz.,  four  hundred  million  millions  in 
a  second.  As  they  become  still  more  rapid  they  appear 
as  different  colors  until  they  reach  about  eight  hundred 
million  millions,  when  they  again  disappear  from  con- 
sciousness. We  know  that  they  are  there,  for  they  show 
themselves  in  other  ways — e.  g.,  by  photography — but 
we  have  no  nerve  specialized  to  perceive  them. 

2.  Kind  of  Contact. — Feeling  takes  cognizance  of 
any  kind  of  direct  material  contact,  but  especially  of 
solid  contact.  Taste  must  have  liquid  contact,  and  un- 
less a  substance  is  soluble  it  can  not  be  tasted.  In  the 
case  of  smell  the  contact  must  be  more  refined  ;  it  must 
ht  gaseous,  and  unless  a  substance  is  volatile  it  can  not 
be  smelled.  In  the  case  of  hearing  and  sight  there  is  no 
direct  contact  at  all  of  the  sensible  body,  the  impres- 
sion being  made  by  vibrations  of  a  medium,  the  air  in 
case  of  hearing  and  the  universal  ether  in  case  of  sight. 
All  other  senses  are  terrestrial ;  sight  alone  is  cosmical. 

3.  Objectiveness. — In  the  three  so-called  lower 
senses,  viz.,  feeling,  taste,  and  smell,  we  are  conscious 
that  the  sensation  is  in  us — i.  e.,  subjective — although  in  . 
smell  we  already  refer  the  sensible  body  to  a  distance. 
In  hearing,  the  thing  perceived  no  longer  seems  to  be  in 
us,  but  in  the  sensible  body — in  yonder  bell — though  there 
may  be  still  a  remnant  of  the  subjective  element.  Final- 
ly, in  sight  there  is  not  a  remnant  of  perception  of  any- 
thing in  us.  The  sensation,  say  of  redness  or  blackness, 
is  completely  externalized,  objectified,  referred  outward 
to  the  sensible  body.  To  the  untrained  mind  there  is 
not  a  suspicion  of  any  change  in  the  eye. 


98 


PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


Higher  and  Lower  Senses. — The  lower  senses 
are  those  in  which  contact  is  direct — viz.,  feeling,  taste, 
and  smell.  The  higher  senses  are  those  in  which  the  im- 
pression is  through  the  vibrations  of  a  medium,  viz., 
hearing  and  sight.  When  the  impression  is  indirect — i.  e. 
through  a  medium — besides  the  specialized  nerve  there 
is  also  a  mechanical  instrument — acoustic  in  the  case  of 
the  ear  and  optic  in  the  case  of  the  eye — placed  in  front 
of  the  specialized  nerve  in  order  to  make  the  impression 
stronger  and  more  definite.  The  specialized  nerve,  to- 
gether with  the  mechanical  instrument,  constitute  the 
sense  organ.  On  these  two  higher  senses,  therefore,  are 
founded  all  our  fine  art  and  nearly  all  our  science. 

SENSE    OF    SIGHT    AND    ITS    ORGAN,   THE    EYE. 

The  most  specialized  of  all  sensory  fibers  are  those  of 
the  optic  nerve  ;  the  most  refined  of  aU  mstruments  is  the 
eye;  the  highest  of  all  the  senses  is  the  sense  of  sight. 
And  yet  we  are  apt  to  greatly  overestimate  what  is 
really  given  by  this  sense.  The  direct  gifts  of  sight  are 
light,  its  intensity,  its  color,  and  its  direction,  nothing  more. 
All  else,  as  size,  distance,  relief -form,  etc.,  are  judgments. 
This  will  be  explained  hereafter. 

Again,  we  must  distinguish  between  light  objective  z.x\ A 
light  subjective.  The  one  consists  of  vibrations  of  the  ether 
and  exist  independent  of  us  ;  the  other  is  the  peculiar  sen- 
sation produced  in  us  or  in  some  other  percipient  by  these 
vibrations.  The  one  belongs  to  physics,  the  other  to 
physiology,  or  perhaps,  some  may  claim,  to  psychology. 
We  are  here  concerned  mainly  with  light  as  sensation  ; 
but  since  this  is  produced  by  vibrations,  we  shall  be  com- 
pelled to  say  something  of  the  physics  of  the  subject  also. 

Primary  Divisions  of  the  Subject.— The  phenom- 
ena of  vision  may  be  divided  into  two  primary  groups: 
monocular    and    binocular.       Monocular    visi(;n    includes 


SENSE   ORGANS. 


99 


all  those  phenomena  which  are  essential  and  universal ; 
binocular  vision  only  certain  additional  phenomena 
which  are  the  result  of  the  use  of  two  eyes  as  one  instru- 
ment. We  take  up,  first,  the  general  phenomena  char- 
acterizing all  vision — i.  e.,  monocular  vision.  And  as  it 
is  impossible  to  understand  these  without  a  knowledge 
of  the  structure  of  the  eye,  this  must  be  our  first  subject. 


SECTION    II. 
The  Eye  of  Man — General  Structure. 

Shape,  Setting,  etc. — The  eye  is  a  nearly  globular 
organ,  about  one  inch  in  diameter,  a  little  more  pro- 
tuberant in  front.  Set  in  a  deep  conical  socket,  it  occu- 
pies only  the  anterior  part;  the  posterior  is  filled  with  a 
cushion  of  fat  on  w'hich  it  rests  and  rolls  easily  in  all 
directions.  The  exposed  part  may  be 
covered  by  the  lids,  which  protect  and 
at  the  same  time  wipe  and  keep  it  clean 
and  bright.  There  is  really  no  inter- 
ruption of  the  skin  here ;  on  the  con- 
trary, the  skin  passes  over  the  lid,  then 
over  its  edge,  then  under  the  lid  as  mu- 
cous membrane,  then  much  short  of  the 
equator  of  the  ball  it  is  reflected  on  to 
the  ball  ((/)  (Fig.  70),  then  over  the 
white  of  the  eye,  then  over  the  clear 
part  (but  here  it  is  very  closely  adherent  and  is  trans- 
parent), and  so  on  to  be  again  reflected  on  to  the  lower 
lid  (at  a')  and  out  on  to  the  face.  By  nice  dissection  it 
is  possible  to  separate  it  continuously,  so  as  to  leave  the 
ball  behind  the  skin.  Now  all  this  tender  portion  lining 
the  lids  and  covering  the  front  part  of  the  ball  is  called 
the  conjunctiva.  Ordinary  inflammation  of  the  eye  is  an 
inflammation   of    this   membrane.     Inflammation  of  the 


Fig.  70. 


lOO   PHYSIOLOGY   AND   MORPHOLOGY   OF   ANIMALS. 


eyeball  itself  is  a  much  more  serious  affair.  From  this 
arrangement  it  is  evident  that  motes  in  the  eye  can  not 
go  beyond  easy  removal,  as  it  can  not  go  beyond  a  a'. 
Fig.  70. 

Muscles. — The  nimble  movements  of  the  eye  are 
effected  by  six  muscles  in  each  eye.  Four  of  these,  the 
straight  muscles  {recti),  arise  near  together  at  the  bottom 
of  the  conical  socket  (Fig.  71),  come  forward  diverging, 
and   are  attached  to   the   ball  a   little    in  front   of   the 

equator,  one  above 
s??^^^!  .1/  (superior  rectus), 
one  below  (inferior 
rectus),  one  on  the 
outside  (exterior 
rectus),  and  one  on 
the  inside  (inte- 
rior rectus).  The 
functions  of  these 
are  obvious.  Each 
turns  the  ball  in  the 
direction  of  its  pull.  When  we  look  upward  the  two 
superior  recti  pull ;  when  we  look  downward,  the  two 
inferior  recti.  When  we  look  to  the  right,  the  external 
rectus  of  the  right  eye  and  internal  rectus  of  the  left 
eye  pull,  and  vice  versa  when  we  look  to  the  left.  When 
we  look  at  a  very  near  object,  then  the  two  interior  recti 
pull  so  as  to  converge  the  eyes  on  the  object  looked  at. 
But  we  can  not  contract  the  two  exterior  recti  so  as  to 
turn  both  eyes  outward,  nor  can  we  turn  one  eye  upward 
and  the  other  downward,  because  these  movements  can 
not  serve  any  useful  purpose,  and  therefore  have  never 
been  learned. 

There  are  two  other  muscles — the  oblicjite.  The  supe- 
rior oblique  arises  along  with  the  recti  at  the  bottom  of 
the  socket,  passes  forward  to  the  opening  of  the  orbit, 


Fig.  71. — Muscles  of  the  eyeball  :  a,  optic  nerve  ; 
b,  superior  oblique  muscle  ;  c,  pulley  ;  d,  in- 
ferior oblique.     The  other  four  are  the  recti. 


SENSE   ORGANS.  lOI 

then  through  a  loop  on  the  inside  which  acts  Hke  a  pul- 
ley, then  back  obliquely  over  the  upper  side  of  the  ball, 
to  attach  itself  on  the  outside  a  little  behind  the  equator. 
Its  pull  being  from  the  loop,  it  turns  the  eye  downward 
and  outward  and  rotates  it  on  its  axis  inward.  The  in- 
ferior oblique  arises  from  the  lower  part  of  the  inside  of 
the  orbital  opening,  runs  under  the  eye  obliquely  back- 
ward and  outward  across  the  equator,  to  attach  itself  on 
the  outside  of  the  ball  a  little  behind  the  equator.  Its 
action,  therefore,  is  to  turn  the  eye  outward  and  up- 
ward and  rotate  it  on  its  axis  outward. 

The  use  of  these  oblique  muscles  is  much  more  ob- 
scure; their  full  explanation  would  carry  us  too  far_ 
For  this  we  would  refer  the  reader  to  the  author's  book 
on  Sight  (pages  ii  and  12  and  page  210). 

In  the  normal  condition,  looking  forward,  the  axes 
of  the  eyes  are  either  parallel  or  equally  convergent,  so 
as  to  bring  their  axes  together  on  the  object  looked  at. 
Any  deviation  from  this  position  is  quickly  detected  by 
an  observer  as  a  squint  or  a  cast.  These  malpositions 
of  the  eyes  are  often,  but  not  always,  caused  by  too. 
great  action  of  some  one  of  the  muscles,  and  are  cor- 
rected by  cutting  the  muscle  and  allowing  it  to  attach 
itself  to  a  new  point  on  the  ball. 

Coats  of  the  Ball. — Take  the  ball  out  of  the  socket. 
Dissect  away  the  muscles.  THe  ball,  except  the  front 
part,  is  seen  to  be  invested  with  a  strong  white  coat  of 
fibrous  tissue.  This  is  the  sclerotic.  It  gives  form  to 
the  eye  and  serves  as  attachment  of  the  muscles.  The 
front  or  more  protuberant  part  is  covered  with  an 
equally  strong  but  perfectly  transparent  coat,  appar- 
ently continuous  with  the  sclerotic.  This  is  the  cornea. 
Its  function  is  to  retain  the  form  of  this  part  of  the  eye, 
and  at  the  same  time  to  freely  admit  the  light.  Look- 
ing through  the  transparent  cornea,  we  see  a  little  way 


102    PHVSIOLOCxY   AND    MORniOLOGY   OF   ANIMALS. 


behind  it  a  flat  colored  screen,  the  iris,  in   the  center  of 
which  is  a  round  hole,  the  pupil,  which  in  the  living  eye 

is    seen   to    expand 
•?    ^  or  contract,  accord- 

ing to  the  intensity 
of  the  light  (F'ig. 
72).  The  color  of 
the  eye  is  the  color 
of  the  iris.  The 
pupil  is  black,  as 
any  hole  opening 
in  a  dark  room  is 
black. 

Linings.  —  For 
further  examina- 
tion dissection  is 
necessary.  We  thus 
find  that  the  sclerot- 
ic part  is  lined  with 

Fig.   72. — Fection  of  the  eye:  O,  optic  nerve;  tWO  membranes.    In 

^,  sclerotic ;  C'//,  choroid ;  i?,  retina ;  t',  vitre-  i-         .  .        ■   1 

ous  body;  Cw,  ciliary  muscle;   Cy,  conjunc-  direct   COntact   With 

tiva;  C,  cornea ;  /ir'is;  /.,  lens;  *  aqueous  ^|         gclerotic     is     a 
humor  ;  ■"■•^,  ciliary  body  or  zonule  of  Zinn. 

dark  brown,  almost 
black,  and  very  vascular  coat,  the  choroid.  Its  function 
is  to  absorb  the  light  as  soon  as  it  strikes.  The  choroid 
lines  the  whole  sclerotic  as  far  forward  as  the  outer 
margin  of  the  cornea.  It  is  there  split  into  two  layers, 
the  anterior  one,  as  it  were,  drawn  together  and  thick- 
ened, forms  the  iris.  The  posterior  one,  also  drawn 
together  and  plaited,  forms  the  ciliary  processes  radiat- 
ing about  the  back  portion  of  the  lens.  The  innermost 
lining  coat  is  the  retina,  the  most  important  of  all.  This 
is  a  deep  cup-shaped  expansion  of  the  optic  nerve.  This 
nerve  enters  the  eve  socket  at  the  bottom,  comes  for- 
ward through  the  middle  of  the  fatty  cushion,  pierces 


SENSE    ORGANS.  I03 

the  sclerotic  and  choroid,  and  then  spreads  as  a  thin 
translucent  membrane  nearly,  but  not  quite,  as  far  for- 
ward as  the  choroid. 

Contents. —  The  hollow  globe  thus  described  is  filled 
with  materials  as  clear  as  finest  glass.  These  are  the 
humors  or  lenses  of  the  eye.  They  are  three  in  num- 
ber. The  crystalline,  or  lens  proper,  is  a  clear,  glassy, 
double  convex  lens,  one  third  of  an  inch  in  diameter 
and  one  sixth  of  an  inch  in  thickness,  and  somewhat  firm 
and  elastic  to  the  touch.  It  is  placed  just  behind  the  iris 
and  in  light  contact  with  it.  It  is  invested  with  a  trans- 
parent membrane,  the  capsule,  which  continues  from  its 
margin  outward  as  a  curtain  to  attach  itself  all  around 
to  the  sclerotic  a  little  behind  the  iris,  and  thus  serves 
to  hold  the  lens  in  its  place.  The  lens  and  the  lens  cur- 
tain divides  the  interior  of  the  eye  into  two  unequal 
parts.  The  smaller  anterior  part  is  filled  with  the  aque- 
ous humor,  the  larger  posterior  part  with  the  vitreous 
humor.  The  aqueous  humor  is  as  liquid  as  water,  the 
vitreous  humor  about  the  consistence  of  soft  jelly.  All 
of  these  may  be  regarded  as  lenses.  The  crystalline, 
with  its  double  convexity,  is  the  lens  par  excellence;  the 
aqueous,  with  its  corneal  surface,  is  also  a  powerful  con- 
vex lens;  while  the  vitreous,  on  its  anterior  surface,  is  a 
concave  lens. 

As  already  said,  all  of  these  are  normally  clear. 
Opacity  of  the  crystalline,  which  often  comes  with  age, 
constitutes  what  is  called  cataract,  and  produces  blind- 
ness. 

FORMATION    OF    THE    IMAGE. 

Now  the  whole  of  what  h.as  been  described  is  an 
elaborate  instrument  to  form  an  image  on  the  sensitive 
retina.  If  we  ask,  "Why  an  image?"  the  answer  is. 
Without  an  image  we  would  perceive  light,  but  not  an  ob- 


104    PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 

ject.  Moreover,  the  image  must  be  an  exact  facsimile  of 
the  object,  because  what  we  see  will  be  a  facsimile  of  the 
actual  image.  See,  then,  the. two  very  distinct  parts  of 
the  eye,  viz.,  the  specialized  nerve  (retina)  and  the  optic 
instrument  for  making  an  image.  The  one  is  necessary 
for  the  perception  of  light,  the  other  for  the  perception 
of  objects. 

In  order  to  bring  out  more  clearly  the  distinctness 
of  these  two  parts  I  will  use  an  illustration.  Suppose 
we  eviscerate  the  eyeball — i.  e.,  remove  all  its  contents — 
leaving  only  a  deep  cup-shaped  cavity  lined  with  the 
retina,  as  can  be,  and  indeed  has  been,  done.  Suppose, 
further,  that  the  retina  could  retain  its  healthy  condition, 
which,  of  course,  is  impossible  but  supposable.  Then 
I  would  undertake  to  have  made  a  glass  eye  which,  fitted 
into  the  cup-shaped  cavity,  would  see  just  as  well  as  the 
natural  eye,  and  perhaps  even  a  little  better. 

The  Necessity  of  Lenses. — The  image  must  be  a 
perfect  facsimile  of  the  object,  because  what  we  see  will 
be  a  perfect  facsimile  of  the  retinal  image.  If  there  is 
710  image  we  will  see  no  object.  If  the  image  is  blurred 
the  object  will  seem  blurred.  If  the  image  is  clear  and 
sharp  the  object  will  be  seen  sharp  in  outline  and  clear 
in  all  surface  details.  Now  light  passing  through  a 
small  hole  will  make  an  image  (pinhole  image),  and 
therefore  a  very  small  pupil  would  make  an  image  on 
the  retina.  But  such  an  image  is  very  imperfect.  To 
make  a  perfect  image  we  must  have  a  lens.  The  man- 
ner in  which  a  lens  acts  in  producing  an  image  is  shown 
in  Fig.  73.  It  is  seen  that  all  the  rays  coming  from 
a  point  {a)  of  the  object  are  bent  (refracted)  in  such  wise 
as  to  meet  one  another  at  a  point  {a')  of  the  image,  all 
the  rays  from  the  point  b  are  gathered  to  one  point  b' , 
and  all  from  c  to  c\  and  similarly  for  all  other  points  of 
the  object.     Thus  for  every  radiant  point  of  the  object 


SENSE   ORGANS. 


105 


there  is  a  corresponding  focal  point  in  the  image,  and 
therefore  the  image  will  be  a  perfect  facsimile  of  the 
object. 

Observe,  then,  (i)  the  image  is  inverted.  All  images 
made  by  lenses  (dioptric  images)  are  inverted.  They 
must  be,  because  the  central  ray  of  the  pencil  from  each 
radiant  passes  straight  through  the  lens  withcnit  bend- 
ing, and  therefore  these  central  rays  all  cross  one  another 


Fig.  73. 

at  a  certain  point  in  the  lens  called  the  nodal  point.  Ob- 
serve again  (2)  that  in  order  to  have  a  sharp  image  the 
receiving  screen  must  be  exactly  at  the  focus  of  rays; 
for  nearer  than  this  the  rays  have  not  yet  come  together 
to  a  focal  point ;  farther  than  this  they  have  already 
crossed  and  spread  out  again.  Observe  (3)  that  the  size 
of  the  image  will  be  to  that  of  the  object  in  the  exact 
proportion  to  their  relative  distances  from  the  nodal  point. 
(4)  Again,  as  the  object  comes  nearer  the  lens  the  image 
will  be  thrown  farther  back,  while  if  the  object  recedes 
from  the  lens  the  image  will  approach  the  lens.  (5)  It 
is  not  every  lens  that  will  make  a  perfect  image.  It 
must  have  a  proper  shape,  and,  moreover,  it  is  found  that 
a  system  of  several  lenses  is  better  than  a  single  lens. 

Application  of  these  Principles  to  the  Eye. — 
Now  the  eye  is  an  instrument  consisting  of  a  system  of 
lenses.  The  eye  therefore  forms  its  images  of  all  ob- 
jects presented  to  it.  In  Fig.  74  rays  from  the  two 
points  A  and  B  of  an  object  .4  B  are  brought  to  focus  on 


lo6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

the  retina  at  a'  b',  and  so  of  all  intermediate  points.  If 
the  retina  is  properly  placed  the  image  will  be  perfect. 
If  the  retina  be  too  far  back  or,  what  is  the  same,  if 
the  lenses  are  too  refractive,  the  image  will  fall  short  of 
the  retina  a'  //  and  will  be  blurred.  This  is  the  case  in 
the  nearsighted.  The  object  must  be  brought  nearer, 
so  as  to  throw  the  image  a  little  farther  back.  If  the 
retina  be  too  near  the  lens  or,  which  is  the  same,  the 
lenses  too  little  refractive,  the  image  will  fall  behind  the 


Fig.  74. — Diagram  illustrating  the  formation  of  an  image  on  the  retina. 


retina  a"  b"  and  will  also  be  blurred.  This  is  the  case  in 
the  old-sighted  for  near  objects.  The  refraction  must 
be  supplemented  by  glasses.  These  defects,  however, 
will  be  explained  later. 

The  fundamental  fact  that  the  images  of  all  external 
objects  are  really  formed  on  the  retina  may  be  shown 
in  many  ways,  (i)  Take  the  dead  eye  of  an  ox.  Re- 
move the  coats  on  the  back  part  of  the  ball,  and  replace 
them  by  a  mica  plate — an  inverted  image  of  the  land- 
scape is  seen.  (2)  The  eyeball  of  a  white  rabbit  shows 
it  without  mutilation,  because  in  these  albinos  the  scle- 
rotic is  more  transparent  and  the  black  pigment  of  the 
choroid  is  wanting.  (3)  The  image  may  be  seen  in  the 
living  eye  by  the  use  of  the  ophthalmoscope. 

It  is  seen  (Fig.  74)  that  the  central  rays  of  the  pen- 
cils cross  one  another  at  the  nodal  point.  In  the  eye 
the  nodal  point  is  a  little  behind  the  center  of  the  lens. 


SENSE   ORGANS. 


107 


The  distance  of  this  point  from  the  retina  is  about  six 
tenths  of  an  inch.  Now  when  we  remember  that  the 
relative  size  of  the  object  and  image  is  exactly  propor- 
tioned to  their  relative  distances  from  the  nodal  point, 
we  at  once  see  how  extremely  minute  the  retinal  images 
of  objects  must  be. 

COMPARISON    OF    THE    EYE    AND    CAMERA. 

The  purely  instrumental  character  of  the  eye  and  its 
mechanical  perfection  may  be  clearly  brought  out  by  a 
comparison  with  the  photographic  camera.  Take,  then, 
the  dead  eye  and  the  dead  camera — i.  e.,  with  only  the 
ground-glass  plate  in  place.  They  are  both  optic  instru- 
ments for  making  an  image.  Look  in  at  the  back  of  the 
camera  and  see  the  inverted  image  on  the  ground-glass 
plate.  Look  in  at  the  back  of  the  eyeball  and  see  also 
the  inverted  images.  Both  are  dark  chambers,  with  a 
lens  in  front  to  admit  the  light  and  make  an  image  by 
refraction  ;  both  are  lined  within  with  black  pigment,  to 
absorb  the  light  and  prevent  reflection  from  side  to  side, 
and  so  the  spoiling  of  the  image.  But  it  is  not  every 
lens  that  will  make  a  perfect  image.  There  are  certain 
defects  in  common  lenses  which  must  be  cor- 
rected to  produce  the  best  effects.  These  are 
corrected  in  the  best  cameras  and  in  all  good 
eyes. 

I.  Chromatism. — Li  all  simple  lenses  we 
find  that  the  images  are  bordered  with  colored 
fringes — blue  or  orange.  These  mar  the  sharp-  y 
ness  of  the  image.  This  defect  is  corrected  in 
optic  instruments  by  a  combination  of  lenses  of  different 
curvatures  and  different  refractive  and  dispersive  pow- 
ers. \\\  a  fine  telescope,  for  instance,  a  double  convex 
lens  is  combined  with  a  plano-concave  lens.  The  one  is 
crown  glass,  the  other  flint  glass  (Fig.  75).     Such  a  com- 


I08    PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


bination  is  achromatic.  Now  the  eye  also  is  corrected 
for  chromatism,  otherwise  all  objects  would  appear 
fringed  with  colors.  The  mode  of  correction  also  is  the 
same,  for  the  eye  also  consists  of  several  lenses  differing 
in  curvature  and  in  material.  In  fact,  the  structure  of 
the  eye  gave  the  first  hint  as  to  the  proper  mode  of 
correcting  lenses — i.  e.,  by  combination. 

2.  Aberration. — In  ordinary  lenses  (i.  e.,  those 
with  spherical  curvature)  it  is  found  that  the  marginal 
rays  are  refracted  too  much  for  the  central  rays,  and 
therefore  all  the  rays  are  not  brought  together  to  the 
same  focus.  This  may  be  partly  remedied  by  cutting 
off  the  marginal  rays  by  a  diaphragm,  but,  of  course, 
with  great  loss  of  light.  But  it  can  be  completely  reme- 
died only  by  making  the  central  part  of 
the  lens  more  refractive.  This  can  be  done 
either  by  graduating  the  density  of  the 
matter  of  the  lens  from  the  margin  to  the 
center,  or  else  by  graduating  the  curvature 
from  margin  to  center.  The  first  method 
art  has  found  impossible  to  accomplish, 
and  therefore  it  adopts  the  second  method. 
Instead  of  a  spherical  curvature,  it  makes 

an  elliptical  curvature,  the  axis  of  the  ellipse  being  the 
a.\is  of  the  lens.  In  this  way  the  best  lenses  are  cor- 
rected for  aberration. 

Now  the  eye  also  is  corrected,  for  otherwise  it  could 
not  sharply  define  the  objects  it  looks  at.  How  is  it  cor- 
rected ?  It  is  probable  that  it  uses  both  methods.  The  crys- 
talline lens  consists  of  concentric  layers,  becoming  denser 
and  denser  to  the  center  (Fig.  76).  Also  the  curvature 
of  the  corneal  surface  is  elliptical  instead  of  spherical. 

3.  Adjustment  for  Distance.  Focal  Adjustment. 
Accommodation. — We  have  seen  that  in  order  to  have  a 
good  image  the  receiving  screen  must  be  at  the  exactly 


Fig.  76. — Sec- 
tion showing 
the  structure 
of  the  lens. 


SENSE    ORGANS.  IOq 

proper  place.  Now  if  the  images  of  all  objects  at  all 
distances  were  thrown  to  the  same  place,  we  might  find 
that  place  and  fix  the  screen  permanently  there.  But 
we  have  already  seen  (page  105)  that  as  the  object  is 
farther  away  the  image  comes  nearer  the  lens,  and  as 
the  object  approaches  the  image  recedes.  Now  there 
are  two  ways  of  adjusting  this  :  if  the  lens  retains  its 
form  then  the  screen  must  be  moved  back  and  forth  to 
the  proper  place,  or  else,  if  the  screen  be  fixed,  the  lens 
must  change  its  form  so  as  to  throw  the  image  on  the 
fixed  screen — i.  e.,  it  must  become  more  refractive  as  the 
object  comes  nearer.  The  former  is  the  method  of  the 
camera  and  of  nearly  all  optic  instruments,  such  as  the 
opera  glass,  the  field  glass,  etc.  In  these  the  tube  is 
drawn  out  so  as  to  carry  the  screen  back  for  near  ob- 
jects, and  is  pushed  in  so  as  to  carry  the  screen  nearer 
the  lens  for  distant  objects.  The  microscope  is  an  ex- 
ception.    Usually,  when  the  object  is  brought  near  (as 

/- 2^ — -iO^ 


Fig.  77. — A,  eye  observed  ;  B,  eye  of  observer  ;  C,  section  of  candle  flame  ; 
/,  a  distant  point  of  sight,  and  n,  a  near  point  of  sight.     (After  Helm- 

holtz.  I 

in  magnifying  greatly)  the  lens  is  changed  and  the  image 
is  thrown  to  the  same  place. 

Now  in  the  eye  the  adjustment  for  distance  is  per- 
fect, for  objects  at  all  distances  from  five  or  six  inches 
to  infinite  distance  ;  for  the  moon  or  sun  is  seen  perfectly 
defined.     How  is  it  done  ?     It  is  done  by  changing  the 


no   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Fig.  78. 


form  of  the  lens.     For  the  complete  proof  of  this  we  are 

indebted   to    Helmholtz.     The   experiment  is   shown  in 

F'g-   77>  ii"*  which  A  is  the  eye  of  the 

•  patient  observed,  B  the  eye  of  the  ob- 
server, and  C  a  candle.  In  looking  into 
the  eye  A  with  an  ophthalmoscope  three 
images  of  the  candle  are  distinctly  seen 
(Fig.  78).  One  of  these — the  first  {a) — is 
the  reflection  from  the  corneal  surface. 
Thesecond  {b),  much  fainter  and  smaller, 
is  from  the  anterior  convex  surface  of  the  lens.  Both  of 
these  are  upright.  The  third  (^),  still  fainter  and  smaller, 
is  inverted,  because  it  is  reflected  from  the  posterior  sur- 
face of  the  crystalline,  which  is  concave.  All  this  is  ob- 
served while  the  patient  gazes  at  a  distant  point  (/, 
Fig.  77).  Now  tell  the  patient  to  look  at  a  very  near 
point  (;/),  perhaps  six  inches  from  the  eye.  Immediately 
the  image  {b,  Fig.  78)  is  seen  to  change.  It  becomes 
smaller,  and  changes  its  place  in  such  wise  as  to  show 
that  the  anterior  surface  of  the  lens  has  become  more 
convex,  has  bulged  out,  and  even  pushed  the  iris  out  a 


Fig.  79. — F^  lens  adjusted  to  distant  objects  ;  N,  to  near  objects  ;  «,  aqueous 
humor  ;  d,  ciliary  muscle  ;  t",  ciliary  process. 


little  (see  Fig.  79).  Thus  it  seems  certain  that  in  accom- 
modation of  the  eye  to  near  vision  the  lens  thickens  and 
becomes  more  refractive.  But  the  question  still  remains, 
How  does  it  do  this  ?  We  are  distinctly  conscious  of  a 
muscular  strain.     What  muscle  ? 


SENSE   ORGANS.  HI 

This  is  not  definitely  settled,  but  we  are  again  in- 
debted to  Helmholtz  for  the  most  probable  view,  viz., 
that  it  is  done  by  contraction  of  the  ciliary  muscle.  W'e 
have  already  mentioned  (page  103)  the  lens  capsule,  its 
continuation  as  a  curtain  outward  all  around,  and  its 
attachment  to  the  sclerotic  a  little  behind  the  iris.  Now 
this  curtain  is  taut,  and  therefore  the  capsule  presses 
gently  on  the  elastic  lens  and  flattens  it.  This  is  the 
passive  condition  of  the  eye  when  it  is  accommodated 
to  distant  objects.  Now  there  is  a  muscular  collar  about 
the  iris,  on  the  inside  of  the  sclerotic,  the  fibers  of  w'hich, 
arising  from  the  outer  margin  of  the  iris,  radiate  out- 
ward and  backward,  and,  taking  hold  of  the  outer  margin 
of  the  lens  curtain  where  it  is  attached  to  the  sclerotic, 
pulls  it  forward  to  where  the  circumference  is  less,  and 
therefore  slackens  its  tautness  and  allows  the  elastic  lens 
to  bulge.  The  amount  of  bulging  is  in  proportion  to 
the  slackening,  which  will  be  in  proportion  to  the  con- 
traction, and  this  in  proportion  to  the  nearness  of  the 
object. 

See,  then  :  the  eye  is  more  like  the  microscope,  in 
that  it  changes  the  lens  rather  than  removes  the  screen. 
But  how  much  more  perfect !  The  microscope  has  its 
four-inch  lens,  its  two-inch  lens,  its  one-inch,  its  half- 
inch,  its  tenth-inch  lens,  and  changes  one  for  another 
as  the  object  is  nearer.  The  eye  has  but  one  lens, 
but  it  changes  the  form  of  its  one  lens  so  as  to  make  it 
a  si.\-inch  lens,  a  foot  lens,  a  twenty-foot  lens,  a  mile 
lens,  or  a  million-mile  lens,  for  at  all  these  distances  it 
makes  a  perfect  image. 

4-  Adjustment  for  Light. — In  both  the  camera  and 
the  eye  some  contrivance  is  wanted  to  regulate  the 
amount  of  light  admitted.  In  both,  too,  this  is  done  by 
diaphragms  with  holes  of  varying  size.  In  the  eye  the 
iris  is  the  diaphragm  and  the  pupil  the  hole.  But  in  this 
9 


112    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

case  the  diaphragm  is  contractile  and    the   pupil  self- 
regulating. 

Structure  of  the  Iris. — The  iris,  as  already  seen,  is  a 
continuation  of  the  choroid,  which  lines  the  sclerotic  as 
far  forward  as  a  little  in  front  of  the  lens  curtain,  and  then 
is  drawn  together  transversely  to  form  the  iris.  This 
part  is  thickened  with  muscular  fibers.  These  are  of 
two  kinds,  circular  and  radiating,  as  shown  in  the  figure 
(Fig.  80).  The  circular  fibers,  by  contracting,  draw  the 
pupil  together  and  make  it  small  ;  the  radiating  fibers 
take  hold  on  the  margin  of  the  pupil  and  pull  it  outward 

in  every  direction  and  en- 
large it.  Or,  perhaps  better, 
regard  the  circular  fibers  as 
sensitive  and  actively  con- 
tractile    and     the     radiating 

FIG.  8o.-Showin|  structure   of        ^^^^^  ^^  ^,^^^j^  ^^^  paSsively 

contractile.  When  the  circu- 
lar fibers  contract  they  draw  up  the  pupil,  stretching  the 
radiating  fibers.  When  they  relax,  the  radiating  fibers 
elastically  contract  and  enlarge  the  pupil.  Now,  the 
circular  fibers  are  in  sympathetic  relation  with  thfe 
retina  in  such  wise  that  stimulation  of  the  retina  by 
strong  light  reflexly  causes  the  pupil  to  contract.  As 
the  light  decreases,  the  pupil  expands  to  take  in  more 
until,  in  the  dark  or  in  case  of  paralysis  of  the  ret- 
ina, the  pupil  expands  until  the  iris  becomes  a  slender 
ring. 

The  hint  has  been  taken  here  also  by  the  instrument 
maker.  The  iris  diaphragm  of  the  microscope  is  made 
of  thin  overlapping  plates  of  steel,  which,  by  turning  a 
thumbscrew,  slide  toward  or  away  from  one  another, 
contracting  or  enlarging  the  opening  between.  It  is 
a  beautiful  contrivance,  but  far  inferior  to  the  liv- 
ing iris. 


SENSE   ORGANS.  u 


DEFECTS    OF    THE    EYE    AS    AN    INSTRUMENT. 

We  have  shown  the  beauty  of  the  eye  as  an  instru- 
ment by  comparing  it  with  the  photographic  camera. 
But  all  eyes  are  not  perfect.  The  defects  of  the  eye  are 
indeed  quite  common,  and  apparently  becoming  more 
and  more  common  through  abuse  of  this  delicate  organ, 
especially  in  the  schoolroom.  In  order  to  understand 
these  defects  it  is  necessary  to  detine  the  normal  eye. 

Normal  Sight— Emmetropy. — The  normal  eye  in 
2l passive  state  is  prearranged  for  a  perfect  image  of  a  dis- 
tant object.  The  focus  of  parallel  rays  is  on  the  retina. 
For  all  nearer  distances  it  accommodates  itself  by  action 
of  the  ciliary  muscles  until  the  object  is  as  near  as  five 
or  six  inches.  Nearer  than  this  it  can  not  accommodate 
itself  to  make  a  perfect  image.  Its  range  of  distinct 
vision,  therefore,  is  from  six  inches  to  infinite  distance. 
This  is  the  standard.  Any  considerable  deviation  from 
this  is  a  defect.  The  most  common  defects  are  niyopy\ 
hyperopy,  prcsbyopy,  and  astii^niatis)/!. 

Myopy,  Brachyopy — Nearsightedness. — This  is 
perhaps  the  most  common  of  all  defects  of  the  eye,  espe- 
cially in  large  cities  and  in  most  advanced  communities. 
In  the  myopic  eye  the  refractive  power  of  the  lenses  of 
the  eye  is  too  great  for  the  position  of  the  retina.  The 
focus  of  parallel  rays  when  the  eye  is  passive  is  not 
on  the  retina,  but  in  front  of  it.  The  rays  must  be  di- 
vergent to  make  a  perfect  image  on  the  retina.  There-, 
fore  distant  objects  can  not  be  seen  distinctly.  The 
object  must  be  brought  near  to  a  certain  limit  before  it 
can  be  seen  well.  But  within  that  limit  it  accommodates 
itself  like  the  normal  eye.  In  the  normal  eye  the  range 
of  distinct  vision  is  from  infinite  distance  to  six  inches; 
in  the  myopic  eye  the  range  is  from  a  yard  to  four 
inches,  or  a  foot  to  three  inches,  or  six  inches  to   two 


114   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

inches,  according  to  the  degree  of  myopy.  The  fault 
being  too  great  refraction,  the  remedy  is,  of  course,  to 
diminish  the  refraction  by  the  use  of  concave  glasses. 
If  these  be  so  chosen  that  the  focus  of  parallel  rays  is 
on  the  retina  when  the  eye  is  passive,  so  that  distant 
objects  are  seen  distinctly  ;  then  the  eye  accommodates 
itself  to  all  nearer  objects,  and  behaves  in  all  respects 
like  a  normal  eye. 

Kyperopy— Oversightedness. — This  is  the  oppo- 
site of  myopy.  In  the  passive  state  the  focus  of  parallel 
rays  is  behind  the  retina.  In  this  defect  the  refractive 
power  of  the  lenses  is  too  small  for  the  position  of  the 
retina.  The  defect  is  far  more  common  than  generally 
supposed.  It  often  exists  unknown  to  the  |)atient  or  his 
friends.  Distant  objects  are  seen  perfectly  well,  because 
a  slight  accommodation  brings  the  focus  on  the  retma. 
But  the  eye  is  never  passive  unless  in  sleep.  For  this  rea- 
son it  is  often  a  distressing  defect,  producing  headaches 
and  the  like.  Since  the  defect  is  a  deficiency  in  refrac- 
tive power,  the  obvious  remedy  is  the  use  of  slightly 
conve.x  glasses  suited  to  the  degree  of  deficiency.  The 
eye  then  functions  exactly  like  a  normal  e3'e. 

Presbyopy— Old-sightedness. — Both  the  preced- 
ing are  structnial  defects;  this  is  a  functional  defect. 
The  eye  may  be  structurally  normal — i.  e.,  in  a  passive 
state  the  focus  of  parallel  rays  is  on  the  retina — but  it 
has  lost  the  power  of  accommodating  itself  to  divergent 
rays.  The  patient  sees  well  distant  objects,  but  can  not 
see  near  objects  well.  In  order  to  see  near  objects  well 
the  eye  must  be  re-enforced  by  convex  glasses.  But 
the  use  cf  glasses  can  not  make  the  eye  normal,  as  in 
the  other  two  defects,  because  it  has  lost  the  accommo- 
dating power.  Therefore  the  glasses  are  not  worn  ha- 
bitually, as  in  the  other  two  defects,  but  only  in  looking 
at  near  objects — not  in  walking,  but  only  in  reading. 


SENSE   ORGANS.  II5 

The  term  longsightedness  or  farsightedness  is  some- 
times used  to  express  this  defect.  It  is  a  misnomer. 
No  eye  can  be  longer-sighted  than  the  young  normal 
eye.  It  can  define  perfectly  the  edge  of  the  moon  or  of 
the  setting  sun.  Moreover,  all  eyes — the  myopic  and 
hyperopic,  as  well  as  the  normal — undergo  the  presby- 
opic change  with  age  ;  but  the  myopic  eye  does  not 
thereby  become  normal,  as  many  suppose. 

Astigmatism — Dim-sightedness.— All  other  eyes 
see  distinctly  at  some  distance,  but  the  astigmatic  eye 
does  not  see  distinctly  at  any  distance.  Hence  the  term 
dim-sightedness.  In  all  other  eyes  all  the  rays  of  light 
issuing  from  a  radiant  point  are  brought  to  a  ioczX  point ; 
in  this  one  they  are  brought  together  to  a  focal  line,  or 
rather  to  two  focal  lines,  one  farther  than  the  other. 
Hence  the  term  astigmatism.*  In  all  other  eyes  the 
curve  of  the  lenses,  and  therefore  their  refraction,  is 
equal  in  all  directions.  In  this  one  the  curve  and  the 
refractive  power  ///  and  down  is  greater  or  less,  usually 
greater,  than  from  side  to  side.  The  remedy  is,  of  course, 
the  use  of  glasses  which  correct  the  unequal  refraction. 
For  example,  suppose  the  curve  and  the  refractive  power 
from  side  to  side  is  normal,  but  the  curve  and  refractive 
power  up  and  down  is  too  great,  then  the  glasses  should 
have  no  curve  horizontally,  but  should  be  concave  ver- 
tically— i.  e.,  should  be  cylindrical  concave  glasses,  with 
the  axes  of  cylinder  horizontal. 

The  usual  test  for  astigmatism  is  a  large  rectangular 
cross,  thus  -\-.  At  a  certain  distance  the  astigmatic  eye 
sees  the  vertical  line  distinctly,  but  the  horizontal  line  is 
blurred.  At  a  certain  other  distance  the  horizontal  line 
is  distinct,  but  the  vertical  blurred.  But  at  no  distance 
are  they  both  distinct. 

*  Not  a  point. 


Il6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


SECTION    III. 

The  Retina  and  its  Functions. 

Thus  far,  as  much  as  possible,  we  have  confined  our- 
selves to  the  eye  as  an  instrument  for  making  an  image, 
and  have  compared  it  with  the  camera  in  order  to  show 
the  beauty  of  its  adaptation  for  that  purpose.  But  in 
both  the  camera  and  the  eye  the  image  is  only  a  means 
to  accomplish  an  end — to  make  a  photogram  in  one  case 
and  accomplish  vision  in  the  other.  In  both  cases  there 
must  be   a  sensitive  screen   to   receive  the  image — the 


Fig.  8i. — A  view  of  the  two  eyes,  with  optic  nerves:  ck^  optic  chiasm; 
rr ,  nerve  roots;  n  and  «',  right  and  left  optic  nerves.  (After  Helm- 
hoitz. ; 

iodized  plate  in  the  one,  and  the  living  retina  in  the 
other.  In  both  cases,  too,  the  most  wonderful  changes 
take  place  in  these  sensitive  screens.     Before  we  can  un- 


SENSE    ORGANS. 


117 


derstand  the  phenomena  of  vision  we  must  know  some- 
thing of  the  general  structure  and  function  of  the  retina. 
Structure  of  the  Retina. — The  second  pair  of  cra- 
nial nerves,  as  already  seen,  arise  by  fibers  partly  from 
the  optic  lobes  and  partly  from  the  thalamus.  These 
fibers  unite  to  form  the  optic  roots  (;-,  Fig.  81),  which  con- 
verge and  unite  to  form  the  chiasm  {c/i).  From  the  chiasm 
there  go  out  diverging  the  two  optic  nerves  («),  which 
enter  the  eye  sockets  near  the  conical  point,  pass  for- 


FlG.  82. — Generalized  section  of  retina,  etc.:  O,  optic  nen-e ;  S,  sclerotic; 
ch,  choroid  ;  R,  retina  ;  /',  baciUary  layer ;  g,  r^anular  and  nuclear 
layer  ;  /,  fibrous  layer  ;  \',  vitreous  humor  ;  c,  central  spot. 


ward  through  the  fatty  cushion  and  between  the  recti 
muscles,  enter  the  eyeballs  a  little  to  the  interior  or 
nasal  side  of  the  a.\is  or  south  pole,  pierce  the  sclerotic 
and  choroid,  and  spread  to  form  the  innermost  lining 
coat  directly  in  contact  with  the  vitreous  humor.  As  a 
thin,  translucent  coat  it  passes  forward  almost  to  the 
attachment  of  the  lens  curtain,  forming  thus  a  deep  cup- 
shaped  receptive  plate  (Fig.  33,  p.  51).  Its  greatest 
thickness  at  the  bottom  of  the  cup  is  one  quarter  milli- 
metre or  one  one-hundredth  inch,  and  thence  thins  out 
to  a  feather  edge  on  the  forward  margin  of  the  cup. 

Although  so  thin,  its  structure  is  very  complex.     In 
a  cross  section  under  a  low  power  of  the  microscope,  it 


Il8    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


is  seen  to  consist  of  three  layers  (Fig.  82) :  (i)  an  inner 
ox  fibrous  layer  in  contact  with  the  vitreous  body,  con- 
sisting wholly  of  in- 
terlacing fibers  of  the 
optic  nerve;  (2)  of  an 
outermost  layer — bacil- 
lary  layer — composed 
entirely  of  rodlike 
bodies  set  on  end  and 
in  contact  with  the 
choroid;  and  (3)  be- 
tween these  a  middle 
layer,  consisting  of 
granules  and  larger 
nucleated  cells,  and 
therefore  called  the 
agranular  and  nuclear 
layer. 

All  three  layers  ex- 
ist in  all  parts  of  the 
retina  except  in  two 
small  spots  :  (i)  where 
the  optic  nerve  en- 
ters there  can  be, 
of  course,  only  the 
fibrous  layer;  (2)  in 
the  center  of  the  bot- 
tom of  the  cup  and  in 
the  very  axis  of  the 
ball   there  is  a  small 

Fig.  83. — Enlarged  section  of  retina  (after  depression  in  which 
Schultze)  :  A,  general  view;  B,  nervous  v,  cu  i 
elements;  a,  bacillary  layer;  b,  interior  the  nDrouS  layer  IS  en- 
limit  of  this  layer;  c,  external  nuclear  [Jrely,  and  the  granu- 
layer ;  a,  external  granular  layer ;  e,  m-  -' '  ° 
temal  nuclear  layer  ;/,  internal  granular  lar  and  nuclear  layer 
layer;  ^,  ganglioniclayer  ; /i,  fibrous  layer,  ,              .      , 

consisting  of  fibers  of  optic  nerve.  nearly  entirely  want- 


SENSE   ORGANS.  1 19 

ing.  This  is  called  the  central  spot  on  account  of  its 
position,  and  the  fovea  on  account  of  its  depression. 

But  the  importance  of  the  retina  is  so  great  that  it 
must  be  studied  more  carefully  under  a  higher  magnifi- 
cation. Fig.  83  is  a  highly  magnified  section.  Concern- 
ing the  inner  or  fibrous  layer  nothing  more  is  revealed. 
The  middle  layer  is  seen  to  be  very  complex,  consisting 
of  several  granular  layers  and  several  layers  of  nucle- 
ated cells  and  one  layer 

of  very  large  ganglion-       u»:^^,^  ^~ 

ic  cells.     The  functions      ^^M^^^       ^M. 
of  these  various  layers 
are  not  certainly  known. 

The    bacillary     layer     is     Fig.  84. — Pacillary   layer,    viewed   from 

the  outside  surface  :   A,  appearance  of 
now  seen  to  contain  two  usual  surfac3;  B,  appearance  of  sur- 

kinds    of    elements— the  ^^"^,  °P'^^  raised  mar -in  of  central 

spot ;  C,  surface  of  central  spot. 

one     slenderer,     longer, 

and  more  rodlike,  the  other  shorter,  stouter,  and  more 
conelike.  The  rods  are  about  one  fourteen-thousandth 
of  an  inch  (one  five-hundred-and-sixtieth  millimetre), 
and  the  cones  about  one  five-thousandth  of  an  inch  (one 
two-hundredth  millimetre)  in  diameter.  The  rods  are 
usually  most  numerous.  Fig.  84  is  a  view  of  the  ouier 
surface,  showing  the  larger  cones  surrounded  by  the 
more  num'=;rous  rods.  But  the  relative  number  of  these 
is  not  the  same  in  all  parts. 

Distribution  of  the  Rods  and  Cones. — On  the 
anterior  margin  of  the  retina  there  are  no  cones,  but 
only  rods.  As  we  approach  the  bottom  of  the  retinal 
cup  the  cones  become  more  and  more  numerous,  and  at 
the  same  time  smaller  until  in  the  central  spot  or  fovea 
there  are  no  rods,  but  only  cones,  and  these  have  become 
very  small,  only  about  one  ten-thousandth  of  an  inch 
(one  four-hundredth  millimetre)  in  diameter  (Fig.  84). 
Further,  it  must  be  observed  that  the  fibers  of  the  fibrous 


120   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


layer — i.  e.,  of  the  optic  nerve — turn  back  through  the 
granular  and  nuclear  layer  and   terminate  in  the  rods 

and  cones  (see  Fig. 

1  83).  These,  there- 
fore, must  be  re- 
garded as  fiber  ter- 
minals   of    the    optic 

H  nerve.  It  is  proba- 
ble that  the  connec- 
tion between  the 
nuclear  cells  and  the 

4  rods  and  cones  is  by 
means  of  dendrites; 
and,  furthermore, 
that  the  dendrites  of 

0  a  nuclear  cell  touch 
those  of  several  xodi%, 
while  the  cells  cor- 

^  responding  to  the 
cones,  especially 
those  of  the  fovea, 

Fig.  85. — Diagram  showing  the  n'.ode  of  con-  .  .   , 

nection  between  nucleated  ce!ls  and  the  rods  COmmuniCate       With 

and  cones:    i,   fibrous  la^er ;  2,  ganglionic  ,^,,„i,       ff^u-^r        n^r 

layer;  3,  first  granular  layer;  4,  first  nuclear  i""^''      lewer,      pci- 

layer;  5,  second  granular  layer;  6,  second  hapS     With    only    one 
nuclear  layer  ;  7,  bacillarv  layer. 

cone,  as  seen  in  the 
accompanying  figure  (Fig.  85).  The  importance  of  this 
will  be  seen  hereafter  (page  131). 

The  Distinctive  Function  of  the  Layers. — The 
function  of  the  fibrous  layer  is  wholly  transmissive.  It 
is  made  up  of  sensory  fibers,  which  transmit  impressions 
on  the  retina  to  the  brain.  The  function  of  the  middle 
layer  is  doubtless  intermediary  between  the  elements  of 
the  bacillary  layer  and  the  fibers  of  the  optic  nerve.  The 
true  receptive  layer  is  the  bacillary.  This  is  proved  (i) 
by  the  fact  that  there  is  only  one  spot  where  this  layer  is 


SENSE   ORGANS.  121 

wanting — viz.,  where  the  optic  nerve  enters — and  this 
spot  is  blind ;  and  (2)  by  the  fact  that  the  central  spot  or 
fovea  is  the  most  sensitive  spot  in  the  retina,  and  there 
the  fibrous  layer  is  entirely,  and  the  middle  layer  almost 
entirely,  wanting.  In  this  spot  the  bacillary  layer  is  al- 
most directly  exposed  to  the  impression  of  light.  Thus, 
then,  the  fovea  is  the  most  highly  organized  spot  of 
the  retina.  It  differs  from  other  parts  in  three  particu- 
lars:   I.  The  bacillary  layer  there  consists  only  of  cones. 

2.  The  cones  there   are  much  smaller  than   elsewhere. 

3.  The  bacillary  layer  is  there  almost  directly  exposed 
to  the  influence  of  light. 

The  distinctive  functions  of  the  rods  and  the  cones 
will  come  up  for  discussion  hereafter.  Suffice  it  to  say 
now  that  the  perception  of  color  seems  to  reside  in  the 
cones  alone. 

Visual  Purple. — There  has  recently  been  found  in 
the  outer  or  terminal  ends  of  the  rods,  but  not  the  cones, 
a  purplish  red  substance,  which  probably  has  an  important 
but  imperfectly  understood  function  in  vision,  and  is 
therefore  called  visual  purple.  It  is  bleached  by  light, 
and  again  restored  by  darkness.  Photographic  images 
(optograms)  of  objects  may  be  taken  on  the  purple  retina 
and  by  appropriate  means  may  be  fixed.*  The  discov- 
ery of  this  substance  naturally  excited  hopes  that  its 
study  would  solve  the  mystery  of  sensation  by  reducing 
it  to  a  chemical  process;  but  these  hopes  have  not  been 
realized,  for  it  is  now  known  that  the  visual  purple  is 
not  present  in  all  animals,  nor  does  it  exist  in  the  cones, 
and  therefore  is  not  present  in  the  fovea,  which  is,  nev- 
ertheless, the  most  sensitive  spot  in  the  retina  both  to 
form  and  color,  though  not  to  simple  faint  light.  The 
visual  purple,  therefore,  is  certainly  not  essential  to  the 
perception  of  either  light  or  color. 

*  Foster's  Physiology,  p.  1254.. 


122    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

There  are,  however,  some  facts  concerning  the  occur- 
rence of  visual  purple  which  throw  light  on  its  function.* 
It  is  wanting  in  night-blind  animals,  such  as  snakes  and 
most  birds,  and  is  abundant  in  nocturnal  animals,  such 
as  most  ruminants  and  all  cats,  and  in  owls  among  birds. 
Its  probable  function  is  to  give  greater  sensitiveness 
to  the  impression  of  simple _/</////,  diffused  light,  but  not 
to  form  and  color,  and  is  therefore  found  in  the  rods, 
but  not  in  the  cones.  It  is  easily  destroyed  by  light 
and  re-formed  in  darkness,  and  is  therefore  specially 
adapted  to  feeble  light.  Hence  in  very  faint  light,  but 
not  in  full  light,  at  night,  but  not  by  day,  we  detect  the 
presence  of  an  object  (though  not  its  form  and  color)  by 
indirect  better  than  by  direci  vision.  Direct  vision  is  by 
the  cones  only  (because  the  image  is  then  on  the  fovea) ; 
indirect  vision  is  by  rods  mostly,  and  these  are  made 
specially  sensitive  by  the  presence  of  the  visual  purple. 
This  explains  also  the  temporary  night  blindness  of  one 
coming  out  from  a  brilliantly  lighted  room  into  the  night. 
The  restoration  of  the  night  vision  is  the  result  of  the 
re-formation  of  the  visual  purple  destroyed  by  the  bril- 
liant light. 

SECTION    IV. 

Perception  of  Space  and  of  Objects  in  Space, 

There  is  a  certain  fundamental  property  of  the  retina, 
optic  nerve,  and  associated  brain  apparatus  which  must 
now  be  explained,  for  it  lies  at  the  very  basis  of  visual 
phenomena. 

The  First  Law  of  Vision.  The  Law  of  Exter- 
nal Reference  of  Retinal  Impressions.  — An  image 
is  formed  on  the  retina,  but  we  do  not  see  the  retinal 
image.     We  do  not  see  anything  in   the  eye,  but  some- 

*  Parinaud,  Rev.  Scientifique,  vol.  iv,  134,  1895. 


SENSE   ORGANS. 


123 


thing — the  object — outside  in  space.  The  object,  how- 
ever, is  the  facsimile  of  the  image.  It  is  as  if  the  retinal 
image  were  [projected  outward  into  space  and  appeared 
there  as  an  external  image,  which  we  interpret  as  an 
object.  I  said  as  if.  Really  the  retinal  image  is  the 
sign  of  the  definiteness  of  the  impression.  For  the 
molecular  changes  in  the  retina  are  graduated  in  degree 
and  kind  exactly  as  the  light  is  graduated.  The  light 
image  is  a  sign  of  an  invisible  molecular  image.  It  is 
this  perfectly  definite  impression,  this  invisible  molecu- 
lar image,  which,  by  the  brain  or  by  the  mind,  is  referred 
outward  into  space  and  interpreted  as  an  object. 

This  law  is  so  fundamental  that  we  stop  a  moment 
to  show  that  it  is  no  new  law  specially  enacted  for  the 
sense  of  sight,  but  only  an  extreme  modification  of  a 
general  law  of  sense-perception. 

Comparison  with  Other  Senses. — We  have  al- 
ready seen  that  stimulation  of  any  sensory  nerve  in  its 
course  is  referred  by  the  brain  to  the /^/v/Z/dra/ extrem- 
ity. If  the  ulnar  or  the  sciatic  nerve  is  pinched  any- 
where in  its  course,  pain  is  felt  not  at  the  place,  but  in 
the  fingers  or  toes  where  the  nerve  is  distributed.  In 
the  case  of  an  amputated  leg,  pinching  the  end  of  the 
nerve  in  the  stump  causes  pain  in  the  toes  where  the 
nerve  is  naturally  distributed,  even  though  there  be  no 
longer  any  foot  at  all.  In  ordinary  sensory  nerves, 
therefore,  stimulation  of  any  part  of  the  nerve  is  referred 
to  the /'^/•//'//^/'a/ extremity.  Now,  the  optic  nerve  differs 
only  in  the  fact  that  the  impression  is  referred  beyond  \.\\^ 
peripheral  extremity  and  out  into  space. 

This  seems  a  great  difference,  but  remember  the  grad- 
ations already  spoken  of  (|)age  96).  In  touch  and  taste 
the  reference  is  only  to  the  peripheral  extremity,  because 
the  necessary  condition  of  sensation  is  direct  contact.  In 
smell  we  have  indeed  a  sensation  in   the  nose,  but  we 


124 


PHYSIOLOGY  AND    MORPHOLOGY    OF   ANIMALS. 


already  refer  it  to  a  distant  body,  because  the  condition 
of  sensation  is  air-borne  particles.  Finally,  in  hearing, 
and  especially  in  sight,  we  lose  entirely  the  sense  of  con- 
tact or  local  impression,  and  refer  it  wholly  into  space. 

Illustrations  of  the  Law. — We  shall  now  try  to 
make  the  law  clear  by  many  illustrative  experiments  : 

Experiment  i. — If  we  could  bare  the  retina  and  touch 
its  surface  we  would  not  feel  it,  but  would  see  di  flash  of 
light — Where  ?  In  space  and  in  a  direction  exactly  oppo- 
site, or  at  right  angles  to,  the  touched  surface.  If  the 
optic  nerve  be  laid  bare  and  pinched  we  would  feel  noth- 
ing, but  would  see  a  flash  of  light  in  space  opposite  that 
part  of  the  retina  where  the  pinched  fibers  are  distributed. 
Of  course  we  can  not  deliberately  make  this  experiment, 
but  the  flash  of  light  is  observed  in  passing  electricity 
through  the  nerve,  and  also  in  cases  of  extirpation  of 
the  eye  at  the  moment  of  rupture  of  the  optic  nerve. 

Experiment  2.  Phosphenes. — Close  the  eyes,  and 
then  press  the  finger  into  a  corner  of  one  of  them.  A 
brilliant-colored  circle  is  seen  in  the  field  of  darkness 
opposite  the  point  pressed.  These  are  called  phos- 
phenes. In  my  own  case  they  are  brilliant  golden  rings, 
with  steel-blue  centers.  They  are  caused  by  the  inden- 
tation of  the  sclerotic  and,  through  it,  the  retina.  But 
any  change  whatever  in  the  retina  shows  itself  as  an 
appearance  in  space. 

Experiment  3.  Miiscce  Volitantes. — Look  at  a  white 
wall,  or  better,  a  bright  sky.  Nearly  all  observers  will  see 
specks  or  clouds  or  tangled  threads  in  the  bright  field, 
slowly  gravitating  downward.  They  are  called  "  muscse 
volitantes,"  or  flying  gnats.  What  are  they  ?  They  are 
slight  imperfections  in  the  vitreous  body.  These  less 
transparent  spots  cast  their  shadows  on  the  retinal  bot- 
tom. But  any  variation  of  the  retinal  surface  shows  itself 
as  an  appearance  in  the  field  of  view  directly  opposite. 


SENSE   ORGANS. 


125 


Fig.  86. — Internal  view  of  the 
retina,  showing  the  retinal 
vessels  ramifying;  over  the 
surface,  but  avoiding  the  cen- 
tral spot.     (After  Cleland.) 


Experiment  4.  Purkinjes  Figjtres. — Darken  the 
room;  close  one  eye,  say  the  left ;  hold  a  lighted  candle 
very  near  the  open  eye,  three  or  four  inches,  and  to  the 
right  side,  so  that  the  retina  is 
strongly  illuminated.  Gaze  on 
the  opposite  wall  until  the  field 
of  view  becomes  darkened  by 
excess  of  light.  Now  move  the 
candle  about,  back  and  forth, 
up  and  down.  Presently  we  set 
a  shadowy  specter  covering  the 
whole  wall,  like  a  great  bodi- 
less spider  with  branching  legs, 
or  a  spectral  tree  with  leafless 
branches.  What  is  it?  It  is 
an  exact  but  greatly  enlarged 
image  of  the  blood  vessels  of  the  retina  (Fig.  86).  These, 
ramifying  in  the  granular  layer,  and  therefore  in  front 
of  the  bacillary  or  receptive  layer,  cast  their  shadows  on 
the  latter.  But  any  change  or  variation  of  this  layer  is 
seen  as  an  appearance  in  space. 

Experiment  5.  Ocular  Spectra — After-images. — Gaze 
steadily  at  the  setting  sun  a  moment,  and  then  turn 
away  and  look  at  the  wall,  the  sky,  or  at  a  distant  build- 
ing. A  colored  image  follows  the  eye  and  is  cast  on 
what  it  looks  at.  Why  so  ?  The  sun's  image  makes  so 
strong  an  impression  on  the  retina  that  it  is  retained  for 
a  considerable  time;  it  makes  a  brand  on  the  retina. 
But  every  change  or  variation  in  the  retina,  whether 
shadow  or  image  or  brand,  shows  itself  as  an  appear- 
ance in  the  field  of  view. 

We  have  taken  the  extreme  case  of  the  sun,  but  any 
bright  object,  such  as  a  candle  flame  in  a  dark  room  or 
a  stained-glass  window,  will  produce  a  similar  effect.  In 
the  case  of   bright   colors,  as  in   stained-glass  windows, 


126   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

the  after-image  is  of  a  complementary  color.  Red  is  seen 
as  green  and  vice  versa;  blue  as  orange-yellow  and  vice 
versa.  But  to  discuss  these  color  phenomena  would 
carry  us  too  far. 

Generalization  of  these  Facts. — We  have  seen 
that  all  changes  or  variations  of  whatever  kind  in  the 
retina,  whether  images  of  objects,  or  shadows,  or  brands, 
show  themselves  as  appearances  in  space.  TJicrefore 
space  itself  as  perceived  by  the  eye  is  an  externalization  or  out- 
ward reference  of  retinal  states.  With  the  eyes  open,  the 
field  of  vieiv  crowded  with  objects  is  the  externalization 
of  the  stimulated  retina  crowded  with  images;  with  the 
eyes  shut,  the  field  of  darkness  is  still  the  externalization 
of  the  unstimulated  retina.  The  sense  of  space  before 
the  eyes  is  ineradicable.  We  can  not  rid  ourselves  of  it. 
In  the  dark  or  with  the  eyes  shut  it  is  still  there,  as  the 
field  of  darkness.  It  is  in  front,  not  behind  the  head. 
It  is  a  positive  appearance,  which  may  be  roughly  out- 
lined and  may  be  described.  It  is  not  black,  but  rather 
dark-gray  or  brown,  with  confused  markings  and  cloud- 
ings and  sometimes  colored  spaces  scattered  through- 
out. Thus  we  have  an  abiding  sense  of  space  as  the  ex- 
ternal representative  of  the  retina,  even  though  we  see 
no  object  in  it,  precisely  as  we  have  an  abiding  sense  of 
a  hand,  even  though  we  feel  nothing  with  it. 

The  Second  Law  of  Vision  ;  the  Law  of  Direc- 
tion.—  The  previous  law  asserts  the  perception  of  space 
as  the  externalization  of  retinal  states,  and  therefore  the 
reference  of  all  retinal  images  to  that  space.  This  law 
gives  the  direction  of  the  external  reference.  In  several 
of  the  preceding  experiments  we  have  alluded  to  their 
general  direction.     We  come  now  to  define  it  exactly. 

The  law  may  be  given  thus:  Any  impression  on  a 
rod  or  cone  of  the  bacillary  layer  is  referred  by  the  rod 
or  cone  back   into  space  end  on — i.e.,  at  right  angles  to 


SENSE   ORGANS. 


127 


h\ 


the  retinal  surface.  Or  it  may  be  otherwise  expressed 
thus  :  The  impression  produced  by  Hght  from  any  radiant 
point  in  space  is 
referred  back  along 
the  ray  line  to  the 
place  whence  it 
came.  Either  of 
these  formulae  is 
sufficient  for  gen- 
eral statement, 
but  the  former  is 
probably  the  more 
exact.  The  lat- 
ter, however,  we  «' 
will  use  for  illus-  ^■ 
tration.  Thus  we 
have  two  con- 
caves, the  spatial 
and  the  retinal ; 
two  worlds,  a 
macrocosm  and  a 
microcosm  ;  and 
these  correspond 
with  one  another 
point  for  point, 
and  exchange 
with  one  another  by  impression  and  by  external  refer- 
ence along  the  lines  connecting  them.  This  is  repre- 
sented in  Fig.  87.  In  this  figure,  of  all  the  rays  proceed- 
ing from  a  radiant  point  and  gathered  by  the  lens  to  a 
focal  point  on  the  retina,  we  take  only  the  central  ray, 
for  this  represents  the  resultant  impression.  These  cen- 
tral rays  cross  one  another  at  the  nodal  point  «.  If  now 
a,  b,  e,  be  three  stars,  then  the  light  from  a  impresses 
the  retma  at  a',  and  is  referred  straight  back  along  the 


Fig.  87. 


-Diagram  representing  corresponding 
points,  retinal  and  spatial. 


128    PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 

ray  line  to  its  proper  place.  IJght  from  b  passes  through 
the  nodal  point  n  and  strikes  a  rod  or  cone  in  the  upper 
part  of  the  retina,  b' ,  and  is  referred  back  along  the  ray- 
line  to  b.  Similarly  light  from  e  passes  a  little  downward 
through  n  and  impresses  the  lower  part  of  the  retina,  and 
is  referred  back  to  its  proper  place  below.  Thus  the 
three  stars  will  be  seen  in  proper  relative  positions.  Thus 
the  lower  part  of  the  retina  corresponds  to  the  upper 
part  of  the  field  of  view,  and  the  upper  part  of  the  retina 
to  the  lower  part  of  the  field ;  the  right  side  of  the  retina 
to  the  left  side  of  the  field,  and  the  left  side  of  the  retina 
to  the  right  side  of  the  field  ;  and  in  each  case  with  the 
utmost  exactness,  point  for  point.  Every  rod  and  cone, 
as  it  were,  knows  its  own  point  in  space,  and  refers  its 
impressions  there. 

Comparison  with  Other  Senses.— Now,  this  is  no 
unique  law  peculiar  to  sight  alone,  but  a  general  law  of 
sense-reference,  though  refined  to  the  last  degree  in  this 
sense.  In  the  case  of  any  impression  on  a  sensitive  sur- 
face the  cause  is  referred  in  a  general  way  along  the  line 
of  impression.  Suppose  we  are  captive,  bound  and  blind- 
folded, and  surrounded  on  all  sides  by  Apache  Indians 
and  a  target  for  their  arrows,  could  we  not  tell  roughly 
the  direction  of  each  shooting  Apache  by  the  direction 
of  the  punch  of  his  arrow?  Now,  every  radiant  is  shoot- 
ing rays  into  the  eye.  Is  it  not  natural  that  every  im- 
pinging ray  should  be  referred  back  along  the  line  of 
its  flight  to  the  point  whence  it  came  ?  Now,  the  retina 
is  specially  and  wonderfully  organized  to  do  this  with 
mathematical  exactness. 

These  two  laws,  the  law  of  spatial  reference  and  the 
law  of  direction,  are  fundamental.  The  one  explains 
why  impressions  made  in  ourselves  are  referred  outward 
into  space.  The  retina,  or  the  brain  through  the  retina, 
creates  visible  space.     The   other   gives   the   direction  of 


SENSE   ORGANS. 


129 


such  reference,  the  exact  place  of  all  objects  and  radiants 
in  space.  Together  they  explain  all  the  phenomena  of 
monocular  vision  except  the  perception  of  color.  With 
this  exception,  the  whole  science  of  monocular  vision 
is  but  an  explication  of  these  two  laws.  All  that  we 
have  further  to  do,  therefore,  is  to  take  up  several 
important  subjects  and  show  how  completely  they  are 
explained  by  these  laws. 

I.    ERECT    VISION. 

Statement  of  the  Problem. — We  have  seen  that 
the  retinal  images  are  inverted.  We  have  also  seen  that 
these  images  are  referred,  as  it  were  projected,  outward 
into  space,  and  are  seen  there  as  external  images,  the 
signs  and  facsimiles  of  the  objects  which  produced  them. 
How  is  it,  then,  that  objects  are  seen  in  their  natural 
positions — i.e.,  erect  I  This  problem  has  puzzled  think- 
ers ever  since  the  inverted  retinal  image  became  known. 

True  Solution. — But  there  is  no  mystery  at  all 
about  it,  if  we  clearly  understand  the  law  of  direction. 
Most  reasoners  on  the  subject  do  not  seem  to  perceive 
that  the  problem  of  erectness  of  objects  does  not  differ 
at  all  from  that  of  seeing  objects  in  their  right  places.  The 
latter  concerns  the  true  position  of  objects,  the  former 
the  true  position  of  radiants.  Objects  are  composed 
wholly  of  radiant  and  retinal  images  of  corresponding 
focal  points.  Now,  if  images  are  referred  each  back 
along  its  ray  line  to  its  proper  place  in  space,  then  also 
the  focal  points  of  these  images  are  similarly  referred 
each  to  its  proper  place.  Now,  as  ray  lines  from  radi- 
ants cross  one  another  at  the  nodal  point  and  thus  in- 
vert the  image,  so  the  reference  lines  recrossat  the  same 
point,  and  thus  reinvert  the  image  in  the  very  act  of  external 
reference.  This  is  shown  in  Fig.  86,  except  that  now  an 
object  replaces  the  stars.    It  is  evident  that  the  two  ends 


130   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

of  the  arrow  must  be  seen,  each  in  its  proper  place,  and 
therefore  erect.  Suppose  we  stand  at  night  in  the  open 
air  beneath  the  star-lit  sky.  Is  it  any  mystery  that  the 
stars  are  seen  each  in  its  proper  place  ?  Now,  every  ob- 
ject consists  of  an  infinite  number  of  starlike  radiants, 
and  each  radiant  is  referred  back  along  its  ray  line  to 
its  proper  place. 

2.    THE    FOVEA    AND    ITS    SPATIAL    REPRESENTATION. 

The  fovea,  or  central  spot,  is  directly  in  the  axis  of 
the  eye — the  south  pole  of  the  globe.  It  differs  from 
other  parts  of  the  retina  in  several  respects:  i.  The 
fibrous  layer  and  the  larger  part  of  the  nuclear  layer  is 
wanting,  so  that  the  bacillary  layer  is  more  directly 
exposed  than  elsewhere  to  the  direct  action  of  light.  2. 
This  part  of  the  bacillary  layer  consists  of  cones  only. 
3.  The  cones  here  are  much  smaller  than  elsewhere. 
From  the  absence  of  the  other  layers  this  point  is 
depressed,  hence  the  name  fovea,  a  pit.  It  is  evidently 
the  most  highly  organized  part  of  the  retina. 

Its  spatial  representative  is  the  spot  we  look  at — the 
point  of  sight,  or  rather  the  line  of  sight,  and  a  small 
area  about  it.  When  we  look  at  anything  the  axis  of  the 
eye  is  turned  directly  upon  it,  and  the  image  of  the  thing 
falls  on  the  fovea.  The  point  we  look  at  and  a  small 
area  about  it  are  seen  distinctly.  If  we  look  steadily 
at  the  point  and  at  the  same  time  observe  our  per- 
ception of  objects  in  other  parts  of  the  field  of  view,  we 
find  that  while  their  presence  is  plain  enough,  their  ex- 
act form  and  surface-detail  are  more  and  more  imper- 
fectly perceived  as  we  go  from  the  point  of  sight.  This 
is  not  the  result  of  imperfect  image,  but  of  imperfect 
perception  of  the  image;  not  the  result  of  an  imperfect 
instrument,  but  imperfect  retinal  response.  We  can  not 
have  a  better  illustration  of  this  than  the  act  of  reading. 


SENSE   ORGANS. 


131 


We  see  perfectly  the  word  we  look  at,  but  words  right 
and  left  are  increasingly  illegible,  and  therefore  we  are 
compelled  to  run  the  eye  along  the  line,  so  that  the  image 
of  every  word  falls  successively  on  the  fovea.  The  eye 
is  the  most  restless  of  organs.  In  looking  at  a  scene  we 
sweep  the  point  of  sight  about  and  gather  up  the  results 
in  memory,  and  thus  seem  to  see  the  whole  scene  dis- 
tinctly. But  really  we  see  distinctly  only  a  very  small 
area.  The  explanation  of  this  is  doubtless  found  in 
retinal  structure.  As  already  seen  (page  120)  the  cones 
of  the  fovea  are  connected  each  with  its  own  fiber, 
whereas  one  fiber  is  connected  with  several  rods. 

Is  this  limitation  of  distinct  vision  a  defect  ?  I  think 
not.  Suppose  we  saw  all  parts  of  the  field  of  view  with 
equal  distinctness:  it  would  be  impossible  to  fi.\  thoughtful 
attention  on  the  thing  looked  at.  But  the  development 
of  the  higher  faculties  of  the  mind  is  conditioned  on  the 
ability  X.o  fix  the  attetition.  Thus  there  are  three  kinds 
or  grades  of  vision  :  i.  Simple  seeing,  which  may  be  un- 
conscious and  involuntary.  2.  Looking,  or  the  volun- 
tary act  of  sight.  3.  Observing,  or  the  thoughtful  act  of 
sight.     This  last  is  characteristic  of  man  alone. 

MinimufH  Visibile. — What  is  the  limit  of  sight  as  to 
smallness  ?  We  answer.  There  is  nothing  so  small  that 
it  can  not  be  seen  if  there  be  light  enough.  A  star  is 
the  nearest  to  a  mathematical  point  that  we  can  well 
conceive.  We  may  magnify  it  three  thousand  diameters, 
and  still  it  is  a  point.  And  yet  a  star  may  be  seen 
plainly  enough.  The  only  sense  in  which  there  is  a 
minimum  visibile  at  all  is  the  smallest  thing  that  can  be 
seen  as  a  magnitude — the  smallest  distance  between  two 
stars  that  they  can  be  seen  as  two,  the  smallest  distance 
between  two  dots  or  two  lines  that  they  can  still  be 
seen  as  two.  This  undoubtedly  depends  on  the  size  of 
the  cones  of  the  fovea.     If  the  images  fall  on  one  cone. 


132    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

the  two  dots  are  seen  as  one;  but  if  far  enough  apart  to 
fall  on  two  cones,  they  are  seen  as  two.  Taking  the 
foveal  cones  as  one  seventy-five-hundredth  of  an  inch 
(one  three-hundredth  millimetre)  in  diameter,  and  the 
nodal  point  as  six  tenths  of  an  inch  (fifteen  millimetres) 
from  the  fovea,  and  the  point  of  sight  as  ten  inches,  the 
niintmujn  visibile  ought  to  be  about  one  four-hundred-and- 
fiftieth  of  an  inch  (one  eighteenth  millimetre).  This  is 
about  the  fact  for  good  eyes.  If  the  point  of  sight  be 
six  inches,  as  it  may  be  in  young  normal  eyes,  the  mini- 
mum will  be  one  seven-hundred-and-fiftieth  of  an  inch 
(one  thirtieth  millimetre). 

Comparison  with  Touch. — The  only  sense  with 
which  we  can  make  comparison  in  this  regard  is  touch, 
because  these  two  are  the  only  senses  that  take  cogni- 
zance of  dimension.  There  is  also  a  minimum  tactile — 
i.  e.,  the  smallest  distance  between  two  tactile  impressions 
in  which  they  can  be  felt  as  tiuo.  This  varies  greatly  in 
different  parts  of  the  body. 

Experiment. — Take  a  pair  of  dividers,  arm  the  points 
with  small  shot  or  bits  of  cork,  so  as  not  to  prick  the 
patient.  Now  try,  on  a  blindfolded  person,  the  distance 
of  separation  between  the  points  when  these  are  felt  as 
two.  It  will  be  found  that  on  the  middle  of  the  back 
the  separation  must  be  two  to  three  inches;  on  the  out- 
side of  the  forearm  or  back  of  the  hand,  about  one  half 
or  three  quarters  of  an  inch  ;  on  the  finger  tips,  about 
one  twelfth  of  an  inch  ;  and  on  the  tip  of  the  tongue, 
about  one  twenty-fifth  of  an  inch,  or  one  millimetre.  In 
the  retina  it  is  one  seventy-five-hundredth  of  an  inch. 

3.    BLIND    SPOT. 

We  have  already  seen  (page  118)  that  there  is  another 
spot  where  all  the  layers  of  the  retina  are  not  present — 
viz.,  just  where  the  optic  nerve  enters  the  eye.     As  the 


SENSE    ORGANS. 


133 


optic  nerve  consists  of  fibers  and  as  it  spreads  these  as 
an  innermost  layer,  all  other  layers  must  be  absent  here. 
Now  as  the  bacillary  layer  is  the  sensitive  layer,  it  fol- 
lows that  the  spot  must  be  blind. 

Experimental  Proof  of  a  Blind  Spot. — Experi- 
ment I. — Make  the  spots  on  a  sheet  of  paper,  a  few 
inches  apart,  thus : 


a  b 

Shut  the  left  eye  and  look  with  the  right  steadily  at 
the  left  figure,  a,  while  the  paper  or  page  is  slowly 
brought  nearer  the  face.  At  a  certain  distance — about 
nine  inches  for  the  above  figures — the  right-hand  figure, 
b,  will  disappear,  but  on  continuing  the  approach  it 
again  reappears.  The  image  of  b  has  traveled  across 
the  blind  spot  and  come  out  on  the  other  side. 

Experiment  2. — Standing  up,  put  a  small  coin  on  the 
table  and  a  finger  of  the  left  hand  beside  it.  Now,  shut- 
ting the  left  eye  and  looking  at  the  finger  with  the  right 
eye,  move  the  finger  slowly  to  the  left  and  follow  it  with 
the  eye.  At  a  certain  point  the  coin  disappears  from  view, 
but  reappears  on  continuing  the  movement  of  the  finger. 

Experiment  3. — Look  at  a  bright  star  with  one  eye, 
say  the  right.  Now  move  the  point  of  sight  slowly  and 
steadily  to  the  left,  horizontally.  At  a  certain  point  the 
star  will  disappear,  only,  however,  to  reappear  on  con- 
tinuing the  movement  of  the  point  of  sight  in  the 
same  direction.  In  this  manner  almost  any  object,  if 
not  too  large,  may  be  made  to  disappear  from  view.  A 
man  sitting  at  a  distance  of.  say,  one  hundred  feet  may 
be  made  to  disappear. 

Diagrammatic  Illustration. — The  condition  under 
which  the  disappearance  occurs  is  represented  in  the 
diagram  (Fig.  88).     The  eyes,  R  and  Z,  are  supposed  to 


J  34 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


come  nearer  to  the  two  objects,  A  and  B,  L  being  shut 
and  R  looking  across  at  A.  The  image  of  A  falls  all 
the  time  on  the  fovea,  a.  The  image  of  £  falls  on  the 
inner  or  nasal  side  of  the  fovea  at  b,  but  not  far  enough 
to  reach  the  entrance  of  the 
optic  nerve,  o  (i?i)  (i).  But  as 
the  face  comes  nearer  the  eye  R 
turns  more  and  more,  the  image 
of  B  falls  first  nea>-  the  place 
(2),  then  on  the  place  (3)  of  the 
entrance  of  the  optic  nerve. 
As  R  approaches  still  nearer 
(4),  the  image  of  B  has  crossed 
and  appeared  on  the  other  or 
nasal  side  of  the  optic  entrance 
and  reappears. 

Spatial  Representative  of 
the  Blind  Spot.— Every  part 
of  the  retina  has  its  representa- 
tive in  the  field  of  view.  There- 
fore the  blind  spot  has  also. 
Why,  then,  do  we  not  see  it  ? 
When  both  eyes  are  open,  of 
course  we  do  not  see  it,  because 
we  see  with  the  one  eye  the  spot 
which  represents  the  blind  spot 
of  the  other  eye.  There  is  no 
place  that  represents  the  blind 
spot  of  both  eyes.  But  even 
with  one  eye  shut  we  see  noth- 
ing. In  fact,  the  expectation  of  seeing  such  a  repre- 
sentative shows  a  misconception.  The  only  true  rep- 
resentative of  a  blind  spot  must  be  an  invisible  spot. 
It  can  not  be  differentiated  from  the  rest  of  the  field. 
Nevertheless   the  place  of  the   representative  of  the 


Fig.  88. 


SENSE    ORGANS. 


135 


blind  spot  can  be  perceived  in  the  field  of  darkness.  At 
night  in  the  dark — when  the  retina  by  long  rest  is  very 
sensitive — if  the  visual  plane  be  lowered  toward  the  feet, 
and  then  the  eyes  be  turned  quickly  and  strongly  to  one 


Fig.  89. — Diag:ram  showing  place  of  the  invisible  spots  in  the  field  of  \ision. 
The  full  lines  show  the  eyes  turned  to  the  right ;  the  dotted  lines  the 
same  turned  to  the  left.     Ps  =  point  of  sight. 

side  or  the  other,  two  brilliant  stars  with  dark  centers 
are  seen  to  flash  out  for  a  moment  in  the  dark  field 
(Fig.  89).  The  phenomenon  is  produced  by  a  pull  on  the 
optic  nerves.  The  dark  center  is  the  spatial  representa- 
tive of  the  blind  spot,  and  the  brilliant  radiating  circle  is 
produced  by  irritation  of  the  surrounding  bacillary  layer. 

COLOR    PERCF.PTION     AND    COLOR-BLINDNESS. 

We  have  thus  far  treated  of  perception  of  light  only 
as  intensity  and  direction.  But  another  primary  per- 
ception is  that  of  color. 


136   PHYSIOLOGY   AND   MORPHOLOGY   OF   ANIMALS. 

Intensity  versus  Color. — As  there  are  two  kinds  of 
perception  of  sound — viz.,  sound  as  simple  sound  or 
noise,  loud  or  faint,  and  sound  as  tone  or  pitch,  high  or 
low,  acute  or  grave — so  there  are  two  kinds  of  percep- 
tion of  light,  viz.,  light  as  simple  light,  bright  or  faint, 
and  light  as  color.  In  both  sound  and  light  the  one  is  a 
question  of  quantity,  the  other  of  quality.  In  both  cases 
the  one  is  a  question  of  strength  of  vibration  or  wave- 
height,  the  other  of  rate  of  vibration  or  wave  length. 
In  both,  too,  there  is  a  limit  to  the  range  of  perception. 
In  the  case  of  sound  the  range  is  great — viz.,  from  the 
lowest,  sixteen  per  second,  to  the  highest,  some  thirty  to 
forty  thousand  per  second,  or  more  than  eleven  octaves. 
In  the  case  of  light  it  is  very  restricted,  four  hundred 
million-million  to  nearly  eight  hundred  million-million, 
or  about  one  octave. 

Primary  versus  Mixed  Colors. — Primary  or  pure 
colorj  are  such  as  are  simple  sensations.  Mixed  or  sec- 
ondary colors  are  such  as  may  be  made  by  mixtures  of 
the  primaries  in  various  proportions.  The  former  are 
few,  the  latter  almost  infinite  in  number.  Both  primary 
and  secondary  colors  may  be  again  mixed  with  black  or 
white,  and  give  rise  to  an  infinite  number  of  shades  of 
each. 

Primary,  Colors. — There  is  much  difference  of 
view  as  to  which  and  how  many  colors  should  be  called 
primary.  Brewster  (and  Newton  before  him)  made 
three — viz.,  red,  yellow,  and  blue,  rejecting  green  because 
it  can  be  made  by  mixing  blue  and  yellow  pigments. 
Young,  and  after  him  Helmholtz  and  nearly  all  physicists, 
make  also  three,  but  they  are  red,  green,  and  violet  or 
blue  approaching  violet,  rejecting  yellow  because  a 
mixture  of  spectral  red  and  spectral  green  makes  a  kind 
of  yellow.  From  the  purely  physical  point  of  view  un- 
doubtly  Helmholtz   and    the    physicists   are  right,   and 


SENSE   ORGANS.  1 37 

Brewster  wrong,  for  pigments  are  never  pure  colors.  A 
mixture  of  blue  and  yellow  pigments  makes  green,  be- 
cause both  of  the  components  contain  some  green  ;  and 
when  they  are  mixed,  the  yellow  and  blue  kill  one 
another,  and  the  green  of  both  comes  out. 

Hering  differs  from  both  the  preceding.  He  makes 
six  primary  colors — viz.,  white,  black,  red,  yellow,  green, 
and  blue.  Furthermore,  according  to  him,  these  con- 
stitute three  pairs  of  complementaries — viz.,  white  and 
black,  red  and  green,  yellow  and  blue.  There  is  but  one 
objection  that  can  be  made  to  Hering's  view — viz.,  his 
inclusion  of  white  and  black.  These  should  be  put  into 
a  different  category — viz.,  that  of  shade  instead  of  color, 
of  intensity  or  quantity  instead  of  quality.  Leaving  out 
these,  Hering's  four  colors,  or  two  pairs  of  complemen- 
taries, are  red  and  green,  yellow  and  blue.  Undoubt- 
edly from  the  point  of  view  of  sensation,  unplagued  by 
any  physical  considerations,  Hering  is  right.  As  color- 
sensations  these  are  perfectly  simple  and  wholly  distinct, 
and  this  is  true  of  no  other  colors.  Scarlet  and  orange 
are  plainly  and  visibly  a  mixture  of  red  and  yellow, 
purple  a  mixture  of  blue  and  red,  and  even  violet  is  a 
blue  with  a  glow  of  red.  White  and  black  are  also  in- 
deed pure  simple  sensations  as  Hering  maintains,  but 
color  is  not  the  proper  word  to  express  these  sensations. 

Theory  of  Color  Perception ;  General  Theory. 
— I.  Color  is  a  simple  sensation  and  incapable  of  analy- 
sis into  any  simpler  elements.  It  must  be,  therefore^ 
the  result  of  retinal  structure.  2.  It  is  an  endowment 
of  the  cones  and  not  of  the  rods.  This  is  shown  by  the 
fact  that  the  distribution  of  color  perception  over  the 
surface  of  the  retina  is  identical  with  the  distribution 
in  number  and  fineness  of  the  cones.  In  the  fovea  there 
is  nothing  but  cones,  and  these  are  very  small,  and  the 
color  perception   is   therefore    keenest   at   the   point   of 


138   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

sight  and  a  small  area  about  it.  As  we  go  outward  from 
the  fovea  in  all  directions,  we  find  the  cones  are  fewer 
and  larger,  until  there  are  none  at  all  on  the  margins  of 
the  retina.  So,  correspondingly,  the  perception  of  color 
is  more  and  more  imperfect  as  we  go  from  the  point  of 
sight  to  the  margins  of  the  field  of  view,  where  it  is 
finally  lost  entirely.  3.  There  must  be  some  response 
of  the  retina  characteristic  of  each  color.  We  may  im- 
agine that  different  cones  are  adapted  to  vibrate  respon- 
sively — co-vibrate — with  different  colors.  Or  we  may 
imagine  different  substances  in  all  the  cones  which  are 
photochemically  affected  each  by  a  particular  color. 
This  latter  seems  the  more  probable  view.  We  shall 
call  such  substances  eolor-stibstances.  Thus  we  have,  say, 
a  red  color-substance,  meaning  not  that  the  substance 
is  red,  but  that  it  is  photochemically  affected  by  a  certain 
rate  of  vibration  and  produces  the  sensation  of  red. 

Special  Theories.  —  Applying  this  to  the  different 
views  as  to  primary  colors,  according  to  Helmholtz,  there 
are  in  the  retinal  cones  three  kinds  of  color-substance 
which  are  responsive  to  three  rates  of  vibration — viz.,  red, 
green,  and  violet  rays,  respectively,  and  these  give  rise 
to  the  corresponding  color  sensations.  If  two  of  them 
are  affected,  they  produce  mixed  colors.  If  all  are 
affected  in  certain  proportions,  we  have  white.  Or,  to 
put  it  another  way  :  pure  colors  affect  only  one,  mixed 
colors  two  or  more,  white  light  all  in  certain  propor- 
tions. According  to  Hering  there  are  only  two  color- 
substances  (three,  if  we  include  white  and  black)  ;  the 
one  by  opposite  affections  produces  the  complementaries 
red  and  green,  the  other  by  opposite  affections  the 
complementaries  yellow  and  blue;  and  the  essential  na- 
ture of  complementariness,  especially  their  mutual  de- 
structiveness,  is  the  necessary  result  of  these  opposite 
affections  of  the  same  substance. 


SENSE    ORGANS. 


139 


Mrs.  Franklin  has  recently  brought  forward  a  view 
which  deserves  and  has  received  much  attention.  She 
thinks  that  color  perception,  like  all  other  faculties,  has 
been  gradually  evolved.  The  steps  were  as  follows: 
First  of  all,  in  the  early  stages  of  evolution  there  was 
but  one  color-substance  in  both  the  rods  and  the  cones 
This  she  calls  gray  color-substance,  because  it  is  photo- 
chemically  affected  by,  and  gives  rise  to,  the  perception 
of  white  and  black  and  all  shades  between — i.  e.,  grays. 
At  that  time  only  white  and  shades,  but  not  colors,  were 
perceived.  Next,  some  of  this  substance  in  the  cones, 
but  not  in  the  rods,  was  differentiated  into  two  color- 
substances — viz.,  yellow  and  blue — which,  separately  af- 
fected, give  rise  to  these  two  colors  respectively,  but 
simultaneously  affected,  to  white  and  shades.  Lastly, 
one  of  these  two — viz.,  yellow — was  again  differentiated 
into  red  and  green  ;  but  these  by  simultaneous  affection 
give  rise  still  to  yellow. 

COLOR-BLINDNESS. 

Many  people  seem  to  discriminate  colors  imperfectly, 
but  only  because  they  do  not  observe  carefully.  They 
see  colors  perfectly  well,  but  have  not  learned  to  name 
them.     This  is  not  color-blindness. 

What  is  Color-Blindness  ? — The  color-blind  do 
not  see  certain  colors  at  all  as  colors,  but  only  as  shades. 
To  take  one  example  :  The  commonest  form  of  color- 
blindness is  that  for  the  colors  red  and  green.  For  such 
a  person  the  red  berries  and  green  leaves  of  a  cherry 
orchard,  or  the  red  carnations  and  the  green  lawn  on 
which  they  grow,  look  much  alike,  and  neither  of  them 
red  or  green,  but  gray.  In  a  word,  they  look  much  as 
a  stereogram  of  the  scene  would  look  to  an  ordinary 
person  looking  through  the  stereoscope ;  for  the  iodized 
plate  is  also  blind  for  these  colors. 


140 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Cause  of  Color-Blindness. — Color-blindness  is  a 
defect  of  retinal  structure.  In  the  case  of  the  color- 
blind one  or  more  of  the  color  substances  are  wanting. 
In  the  red-green  blind,  for  example,  the  red  color-sub- 
stance and  the  green  color-substance  of  Helmholtz  are 
both  wanting.  Or,  according  to  Hering's  better  view,  the 
one  substance  which,  by  opposite  affections,  produces 
these  complementaries,  is  wanting,  and  that  is  the  rea- 
son why  these  two  are  usually  associated.  Such  persons 
see  yellow  and  blue  perfectly  well.  According  to  Mrs. 
Franklin,  color-blindness  is  an  example  of  atavism — 
i.  e.,  a  reversion  to  a  primitive  condition.  Total  color- 
blindness, which,  though  rare,  sometimes  occurs,  is  a 
relapse  to  the  earliest  condition.  There  is  only  gray 
substance  in  the  retina.  Red-green  blindness  is  a  re- 
lapse to  the  second  stage,  in  which  some  of  the  gray 
substance  has  been  differentiated  into  yellow  and  blue, 
but  the  yellow  has  not  been  further  differentiated;  while 
normal  vision  is  the  third  or  perfect  stage,  in  which  the 
yellow  has  been  further  differentiated  into  red  and  green. 

What  the  Color-Blind  really  See. — By  the  color- 
blind////-^ colors  are  either  seen  correctly  or  not  seen  at 
all  as  colors,  but  only  as  shades.  The  mixed  colors  they 
always  see  incorrectly.  Taking  the  commonest  form  of 
color-blindness,  the  red-green  blindness,  the  following 
schedule  shows  what  they  see  and  why : 

I. 

See  correctly. 

a.  White  and  black  and  all  shades  of  the  same — i.  e.,  grays. 

b.  Yellotv  and  all  shades  of  the  same — i.  e.,  browns. 

c.  Blue  and  all  shades  of  the  same — i.  e.,  slate  blues. 

II. 

Do  not  see  at  all  as   Colors. 
a.   Reds  are  seen  as  shades  or  grays. 
,  l>.  Greens  are  seen  as  shades  or  grays. 


SENSE   ORGANS. 


141 


III. 

See  incorrectly, 
'a.   Scarlet  =  red   and  yellow — i.  e.,   gray   and   yellow  =  dark 
brown. 

b.  Orange  =  red  and  yellow — i.  e.,  gray  and  yellow  =  lighter 
brown. 

c.  Bluish  green  =  blue  and  green — i.  e.,  blue  and  gray  =  slate- 
blue. 

d.  Yellowish  green  =  yellow  and  green — i.  e.,  yellow  and  gray 
=  brown. 

e.  Purple  =  red  and  blue — i.  e.,  gray  and  blue  =  slate-blue. 

Tests. — It  might  seem  that  so  striking  a  phenome- 
non needs  no  test.  Every  one  must  know  it.  But  this 
is  far  from  the  fact.  On  the  contrary,  a  man  may  be 
color-blind  unknown  to  himself  and  to  his  friends.  He 
may  have  observed  some  instances  of  curious  confusion 
of  colors,  but  these  are  attributed  to  imperfect  knowledge 
of  color  names.  In  the  case  of  persons  in  responsible 
positions,  such  as  locomotive-engine  drivers,  ship  steers- 
men, etc.,  where  color  signals  are  used,  it  is  very  impor- 
tant that  ability  to  see  colors  correctly  should  be  tested. 
The  simplest  test  and  one  of  the  best  is  a  box  full  of 
skeins  of  yarn  of  all  colors  and  shades,  and  several  of 
each.  Such  a  box  is  placed  before  the  person  to  be 
tested,  and  he  is  directed  to  sort  them  and  match  the 
colors.  All  normal-sighted  people  would  match  them 
alike  and  correctly,  but  the  color-blind  make  the  most 
extraordinary  mistakes.  Certain  shades  of  red  and  green 
and  gray  are  put  together  as  the  same  ;  similarly  certain 
shades  of  scarlet  and  brown  or  purple  and  slate-blue. 

By  these  tests  the  remarkable  fact  is  brought  out 
that  this  defect  is  much  more  common  in  men  than  in 
women.  About  one  in  every  twenty-five  men  are  more 
or  less  color-blind,  while  among  women  hardly  one  in  a 
thousand  is  thus  affected. 


142    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


SECTION  V. 

Binocular  Vision. 

All  the  phenomena  thus  far  treated  are  essential  to 
vision.  They  would  still  be  found  if,  like  the  Cyclops 
Polyphemus,  we  had  but  one  eye  in  the  middle  of  the 
forehead.  But,  in  addition  to  these,  there  are  certain 
other  phenomena  which  are  wholly  the  result  of  the  use 
of  two  eyes  as  one  instrument.  These  belong  to  binocular 
vision.  Observe,  it  is  not  the  mere  having  of  two  eyes 
which  gives  rise  to  these  phenomena.  We  might  have  a 
hundred  eyes  and  have  no  binocular  phenomena,  for 
each  eye  may  act  independently,  as  is  the  case  in  many 
lower  animals.      The  tivo  eyes  must  act  as  one  instrument. 

The  phenomena  now  about  to  be  described  are  far 
more  illusory,  more  psychical,  more  difficult  to  be  ob- 
served. Although  we  are  forming  judgments  based  on 
them  every  day  of  our  lives,  yet  they  usually  drop  out 
of  consciousness,  and  by  many  persons  are  recalled  to 
consciousness  with  difficulty.  For  this  reason  we  shall 
be  compelled  to  treat  them  much  more  cursorily  than 
their  importance  deserves.* 

SINGLE    AND    DOUBLE    VISION. 

Double  Vision. — We  have  two  eyes,  two  retinae,  and 
two  fields  of  view — their  spatial  representatives — though 
they  indeed  partly  overlap  and  form  a  common  field. 
We  have  also  two  retinal  images  of  each  object,  and  two 
external  images,  the  spatial  representatives  of  the  two 
retinal  images.  Why,  then,  do  we  not  see  everything 
double?  So  indeed  we  often  do,  but  without  observing 
it.  It  is  necessary  first  of  all  to  prove  this.  I  do  so  by 
some  simple  experiments. 

*  This  subject  is  fully  treated  in  my  book  Sight. 


SENSE    ORGANS. 


143 


Experiment  i. — Hold  up  the  finger  against  the  op- 
posite wall  or  against  the  sky,  and  look  not  at  the  finger 
but  at  the  wall  or  sky.  Two  finders  are  seen,  shadowy, 
transparent,  because  they  hide  nothing;  the  place  cov- 
ered by  each  is  seen  by  the  other  eye.  While  still  look- 
ing at  the  sky  or  wall,  shut  the  right  eye ;  the  left  image 
disappears.  Shut  the  left  eye ;  the  right  image  disappears. 
Evidently  the  right  image  belongs  to  the  left  eye  and 
the  left  image  to  the  right  eye.  Such  are  called  heter- 
onymously  double  images. 

Experiment  2. — Hold  the  two  forefingers,  one  be- 
fore the  other,  directly  in  front — i.  e.,  in  the  middle  plane 
of  the  head,  and  twelve  to  fifteen  inches  apart.  Look 
at  the  farther  finger;  the  nearer  one  is  double.  Look  at 
the  nearer  finger  ;  the  farther  one  is  double.  By  shutting 
alternately  first  one  eye  and  then  the  other  it  will  be  found 
that  in  the  former  case  the  images  each  belong  to  the 
eye  on  the  opposite  side — i.  e.,  are  heteronymous,  while  in 
the  latter  case  they  belong  each  respectively  to  the  eye 
on  the  same  side.     Such  are  called  homonymous. 

We  might  multiply  experiments  indefinitely,  but  these 
are  sufficient  to  show  that  we  often  see  objects  double. 
They  show  more,  viz.,  that  when  we  look  at  an  object 
we  see  it  single,  but  all  objects  beyond  or  this  side  of 
the  point  of  sight  are  doubled,  but  in  opposite  ways — 
in  the  former  case  homonymously,  in  the  latter  heter- 
onymously.  This  doubling  of  objects  is  evidently  the 
necessary  result  of  the  two  retinal  images.  But  the 
questions  occur  :  Why  should  we  see  objects  single  at 
all  ?  What  are  the  positions  of  the  two  retinal  images 
when  objects  are  seen  single  ? 

Single  Vision. — Since  there  are  two  retinal  images 
of  every  object  and  two  e.xternal  images,  their  spatial 
representatives,  it  is  evident  that  single  vision  can  only 
take  place  when  the  two  external  images  are  superposed nwd. 


144 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


coincide  perfectly ;  and  this  takes  place  when  the  retinal 
images  fall  on  corresponding  points  of  the  two  retinae. 
It  is  necessary  to  define  these  exactly. 

Corresponding  Points. — Corresponding  points  are 
points  exactly  similarly  situated  in  the  two  retinae.  The 
foveae  are,  of  course, /ar  excellence  corresponding  points, 
and  all  other  corresponding  points  are  symmetrically 
arranged  about  these.  If  R  and  L  (Fig.  90)  represent 
projections  of  the  two  retinae,  and  c  c  the  centers  of  the 
foveae,  and  vertical  and  horizontal  lines  be  drawn 
through  the  central  spots,  then  points  similarly  situated 
in  reference  to  these — viz.,  e  e',  d  d' — are  corresponding 
points.     Or,  suppose  the  two  retinae  be  placed  one  on 


Fig.  90. — Diagram  showing  corresponding  halves  of  the  retinae. 


the  Other  in  geometric  coincidence,  then  the  points — 
the  rods  and  cones — which  coincide  are  corresponding 
rods  or  cones.  It  follows  that  the  two  right  or  shaded 
halves  are  corresponding  halves,  and  similarly  the  two 
left  or  unshaded  halves — i.  e.,  points  similarly  situated 
in  the  two  right  halves  or  left  halves — are  correspond- 
ing. But  the  two  inner  or  nasal  halves  have  no  corre- 
sponding points,  nor  have  the  two  external  or  temporal 
halves  any  correspondents. 

The  Third  Law  of  Vision  ;  the  Law  of  Corre- 
sponding Points. — We  restate  now  the  conditions  of 
single  vision  as  a  law.      IVlien  the  two  retinal  images  of 


SENSE   ORGANS. 


H5 


any  object  fall  on  corresponding  points,  then  the  external 
images  are  thrown  to  the  same  place  and  are  superposed 
and  seen  single,  but  when  the  ttvo  retinal  images  of  an 
object  fall   on    non-corresponding  points    then    the    external 


Fig.  91. 


images  are  thrown  to  different  places  and  are  seen  double. 
Now  it  is  at  once  seen  why  we  see  single  what  we  look 
at ;  for  then  the  axes  of  the  two  eyeballs  are  converged 
on  the  object  and  the  images  fall  on  the  central  spots 
or  foveae,    and    these   are  par  excellence   corresponding 


146   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

points.  Why  we  see  double  all  objects  nearer  and 
farther  off  than  the  point  of  sight,  and  differently 
double  in  the  two  cases,  is  shown  in  the  diagram 
(Fig.  91).  Let  A,  B,  and  C  be  three  objects  in  the 
median  plane,  and  the  eyesi?  and  Z  be  fixed  on  A.  The 
images  of  A  will  fall  on  the  central  spots  and  be  seen 
single;  but  the  images  of  B  will  fall  on  the  two  nasal 
halves,  b  b\  but  all  points  in  these  are  non-correspondent 
and  therefore  B  will  be  seen  double.  Similarly  C  will 
be  seen  double  because  its  images  fall  on  the  two  tem- 
poral halves.  The  kind  of  doubling  in  each  case  may 
be  shown  by  referring  all  the  external  images  to  the 
plane  of  sight,  P  P.  It  is  then  seen  that  the  images 
bb'  oi  B  are  homonymous,  while  the  images  c  c'  oi  C  are 
heteronymous.  That  is,  as  we  before  found,  objects 
nearer  than  the  point  of  sight  are  doubled  heterony- 
mously  while  objects  farther  than  the  point  of  sight  are 
doubled  homonymously. 

Horopteric  Circle. — As  already  shown,  objects  be- 
yond or  on  this  side  of  the  point  of  sight  are  seen  double. 
But  how  is  it  with  points  about  the  same  distance,  but 
right  or  left,  or  above  or  below  that  point  ?  Take  first 
right  and  left.  Let  R  and  L  (Fig.  92)  be  the  two  eyes 
and  A  the  point  of  sight.  Draw  a  circle  through  A  and 
through  the  nodal  points  n  n' .  This  is  the  horopteric 
circle,  or  circle  of  single  vision,  of  Miiller.  For  if  the 
eyes  be  fixed  on  A^  any  object  at  that  point  will  be  seen 
single  because  its  images  are  on  the  central  spots  a  a', 
but  at  the  same  time  B  or  any  other  point  in  the  circle 
will  also  be  seen  single  because  its  images  will  fall  on 
bb\  which  are  obviously  corresponding  points.  But  this 
is  not  true  of  any  point  B'  in  the  plane  P  P. 

Horopter. — We  have  taken  points  right  and  left. 
If  there  be  also  points  above  and  below  seen  single  at 
the   same   time,  then  there  would   be  a  surface  of  single 


SENSE   ORGANS. 


147 


vision.  Such  a  supposed  surface  of  single  vision  with  the 
point  of  sight  fixed  \%  cdiW&d  the  horopter.  Whether  there  be 
such  a  surface  at   all,  and   if  there  be,  what  is  its  form, 


Fig.  92. — The  horopteric  circle  of  MuUer  :  R  and  L,  two  eyes  ;  n  n\  point 
of  crossinf^  of  ray  lines — nodal  point ;  A,  point  of  sight ;  B,  some  other 
point  in  the  horopteric  circle  A  nn  \  a  a',  central  spots;  iia\  bb' ,  ret- 
inal images  of  A  and  B. 


are  very  complex  and  difificult   questions  which  can  not 
be  discussed  here.* 

The  Relation  of  the  Chiasm  to  Corresponding 
Points. — The    union    of    the   optic    nerves    to    form   a 

*  They  are  fully  discussed  in  author's  book  Sight. 


148    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

chiasm  (Fig.  81,  page  116)  is  undoubtedly  related  in 
some  way  with  the  use  of  the  two  eyes  as  one  instru- 
ment, and  therefore  with  the  existence  of  corresponding 
points.  The  fibers  of  the  optic  roots  partly  cross  and 
partly  do  not  cross,  as  shown  in  the  diagram  (Fig.  93). 


Fig.  93. — O  0\  optic  roots ;  NN\  optic  nerves ;  R  and  Z,  sections  of  the 
two  eyes;  c  c' ,  central  spots;  nn',  the  nasal  halves,  and  1 1\  the  tem- 
poral halves,  of  the  retinae. 

Thus  each  root  supplies  both  eyes,  and  conversely  each  eye 
is  controlled  by  both  sides  of  the  brain.  The  existence  of  a 
true  chiasm  with  fibers  crossed  in  this  peculiar  way  may 
therefore  be  taken  as  evidence  of  the  existence  of  corre- 
spondinof  points  and  the  possession  of  binocular  vision. 

The  Two  Adjustments  of  the  Eyes. — There  are 
two  fundamental  adjustments  of  the  eyes  in  every  act 
of  looking,  viz.,  the  focal  adjustment,  or  accommoda- 
tion, and  the  axial  adjustment,  or  turning  the  axes  so  as 
to  converge  on  the  object  looked  at.  The  one  is  neces- 
sary for  distinct  vision,  the  other  for  single  vision.  Asso- 
ciated with  these,  but  far  less  important,  is  a  third,  viz., 
pupillary  contraction. 

Two  Kinds  of  Corresponding  Points.— We  have 
already  (page  127)  spoken  of  corresponding  points,  ret- 


SENSE   ORGANS. 


149 


inal  and  spatial.  We  have  just  explained  the  correspond- 
ing points  of  the  two  retinae.  Now  we  assert  that  the 
corresponding  points  in  the  two  retina  have  the  same  spatial 
correspondent.  So  that  there  is  a  kind  of  triangular 
correspondence  between  the  two  eyes  and  space. 

The  Two  Fundamental  Laws  of  Vision.— There 
are  also,  as. we  have  seen,  two  fundamental  laws  of 
vision — the  law  of  direction  and  the  law  of  corresponding 
points.  The  one  explains  the  apparent  anomaly  oi  erect 
vision  with  inverted  retinal  image,  the  other  the  appar- 
ent anomaly  of  single  vision  with  two  retinal  images. 
The  one  is  the  fundamental  law  of  monocular,  the  other 
of  binocular  vision.  We  have  seen  how  all  the  phenom- 
ena of  monocular  vision  flow  logically  from  the  one. 
Now  we  proceed  to  show  how  all  the  phenomena  of 
binocular  vision  follow  necessarily  from  the  other. 
There  is,  however,  a  third  law  underlying  both  and  more 
fundamental  than  either — viz.,  the  law  of  outward  or 
spatial  reference  of  all  retinal  states. 

BINOCULAR    PERSPECTIVE. 

The  law  of  external  reference  gives  space.  The  law 
of  direction  gives  two  dimensions  of  space — i.  e.,  up  and 
down  and  from  side  to  side.  Now,  the  law  of  corre- 
sponding points  gives  the  third  dimension  of  space — i.  e., 
depth  or  distance  from  the  observer.  The  perception 
of  this  third  dimension,  so  far  as  it  is  dependent  on  the 
use  of  the  two  eyes  as  one  instrument,  is  our  next  sub- 
ject.    We  begin  again  with  experiments  : 

Experiment  i. — We  repeat  that  given  on  page  143, 
but  for  another  purpose.  Place  the  two  forefingers,  one 
before  the  other,  in  the  median  plane,  and  separated,  say, 
a  foot  from  one  another.  We  have  already  shown  that 
when  we  look  at  the  nearer  finger  we  see  it  single,  but 
the  farther  finger  is  doubled  homonymously.     When  we 


I50 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


look  at  the  farther  finger  we  see  that  one  single;  but 
now  the  nearer  one  is  doubled  heteronymously.  Now 
observe,  further,  that  we  are  clearly  conscious  that  it 
requires  more  convergence,  and  therefore  more  effort, 
to  look  at  the  nearer  finger  and  see  it  single,  and  less 
convergence  and  less  effort  of  the  ocular  muscles  to 
look  at  the  farther  finger  and  see  that  .single.  In 
other  words,  we  run  the  point  of  sight  back  and  forth 
from  one  finger  to  the  other  by  greater  and  less  con- 
vergence, and  thus  acquire  a  distinct  perception  of  dis- 
tance between  the  two.  It  is  literally  a  process  of  rapid 
triangulation,  with  the  interocular  distance  as  the  base 
line.  The  same  is  true  of  all  objects  in  space  at  differ- 
ent distances  if  the  distance  of  the  nearer  one  be  not 
too  great. 

Experiment  2. — But  single  objects  also  occupy 
depth  of  space.  Take,  therefore,  next  a  rod,  say  a  foot 
long;  hold  in  the  median  plane,  a  little  below  the  hori- 
zontal line,  with  the  nearer  end  six  to  eight  inches  from 
the  face.  Looked  at  with  one  eye,  say  the  right,  the 
rod  is  seen  projected  thus    /  ;  looked  at  with  the  left 

eye,  \  .  Now,  it  is  evident  that  these  two  images  can 
not  combine.  When  we  open  both  eyes  and  look  at  the 
farther  end,  the  nearer  end  is  doubled  heteronymously, 
and  we  see  the  rod  as  an  inverted  V,  with  the  open  end 
toward  us,  thus  /K  ;  when  we  look  at  the  nearer  end, 
the  farther  end  is  doubled  homonymously,  and  we  see  a 
V  with  the  point  toward  us,  thus  \J  ;  when  we  look  at 
the  middle,  we  see  the  two  images  cross  in  the  middle  to 
make  an  X,  thus  V  .  Thus  we  run  the  point  of  sight 
back  and  forth  from  one  end  to  the  other,  by  greater 
and  less  convergence  uniting  each  point  looked  at,  and 
acquire  thus  a  distinct   perception  of  the  distance  be- 


SENSE    ORGANS.  151 

tween  the  two  ends.  The  same  is  true  of  all  objects 
occupying  depth  of  space. 

Thus,  then,  we  may  safely  generalize  :  In  viewing  a 
single  object  occupying  considerable  depth  of  space,  or 
a  scene  with  objects  one  beyond  the  other,  it  is  evident 
that  the  retinal  images  of  the  object  or  of  the  scene  in 
the  two  eyes,  and  therefore  the  external  images — their 
spatial  representatives — or  the  way  the  object  or  scene 
looks  to  the  two  eyes,  respectively,  are  different,  because 
taken  from  differe?it  points  of  view.  Therefore  they  can 
not  be  united  as  a  whole,  but  only  in  parts  at  a  time. 
When  we  look  at  the  foreground,  objects  in  the  back- 
ground are  double  ;  when  we  look  at  the  background 
objects  in  the  foreground  are  double.  Thus  we  run  the 
the  point  of  sight  back  and  forth,  uniting  successively 
different  parts  of  the  scene,  and  acquire  thus  a  clear 
perception  of  depth  of  space  between. 

Limitation  of  Clear  Vision. — See,  then,  the  ex- 
treme limitation  of  distinct  vision  and  of  single  vision. 
As  distinct  vision  is  confined  to  a  small  area  about  the 
point  of  sight,  and  we  must  therefore  sweep  about  this 
point  and  gather  up  the  result  in  memory,  even  so 
single  vision  is  limited  to  the  distance  of  the  point  of 
sight,  and  we  must  run  the  point  of  sight  back  and  forth, 
uniting  successively  different  parts  o^  the  scene,  thus 
probing  space  and  gauging  its  depth,  and  gather  up  the 
results  in  memory. 

Different  Forms  of  Perspective. — Of  course,  there 
are  other  ways  of  judging  of  relative  distance — other 
forms  of  perspective.  It  may  be  well,  therefore,  to  give 
these,  and  very  briefly  compare  them  : 

I.  Aerial  Perspective. — We  judge  of  distance  by  the 
color  of  the  air  through  which  we  look.  The  atmos- 
phere is  not  absolutely  transparent,  but  bluish.  Distant 
objects,  like  mountains,  are  dimmer  and   bluer  in   pro- 


152 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


portion   to  their  distance,  and  we  judge  of  distance  in 
this  way. 

2.  Mathematical  Perspective. — The  angular  diameter 
of  objects,  and  therefore  the  size  of  the  retinal  image,  is 
mathematically  proportioned  to  the  distance.  Therefore 
objects  seem  smalMn  proportion  to  their  distance.  Par- 
allel lines,  like  railway  tracks,  converge,  and  houses  on 
the  two  sides  of  a  street  converge  and  grow  smaller 
with  distance.  We  judge  of  distance  quite  accurately  in 
this  way. 

3.  Binocular  Perspective. — This,  as  already  explained, 
is  a  judgment  of  distance  by  running  the  point  of  sight 
back  and  forth,  successively  uniting  double  images  by 
greater  and  less  convergence,  and  thus  gauging  space. 

4.  Focal  Perspective. — When  with  one  eye  we  look  at 
a  very  near  object,  farther  ones  are  dim,  and  vice  versa. 
We  are  aware  of  voluntary  effort  of  accommodation 
for  distinct  vision  of  near  objects,  and  judge  of  relative 
distance  in  this  way  also. 

Distance  at  which  these  Operate. — Now,  of  these 
four  kinds,  the  focal  operates  for  only  about  twenty 
feet.  Beyond  this  the  accommodation  is  a  vanishing 
quantity.  The  binocular  perspective  operates  for  about 
one  quarter  to  one  half  mile.  Beyond  this  it,  too,  be- 
comes a  vanishing  quantity.  The  other  two  operate 
without  limit. 

The  painter  can  imitate  the  first  and  second,  and  much 
of  his  art  consists  in  skillfully  introducing  an  appear- 
ance of  distance  by  dimming  and  bluing  and  making 
smaller  the  objects  in  the  background'  of  his  picture. 
The  other  two  he  can  not  imitate.  The  lack  of  focal 
perspective  is,  however,  of  little  importance,  because 
landscape  pictures  are  usually  viewed  at  a  consider- 
able distance.  But  the  lack  of  binocular  perspective 
seriously  interferes  with  the  illusion  which  he  seeks  to 


SENSE   ORGANS. 


153 


produce.      Hence  the   perspective  is    far   clearer  wlien 
the  picture  is  looked  at  with  one  eye  only. 


JUDGMENTS    OF    SIZE    AND    DISTANCE. 

The  eye  perceives  at  once  direction  up  and  down  and 
right  and  left,  and  therefore  outline  form  and  surface 
contents,  for  this  is  a  combination  of  directions.  Thus 
two  dimensions  of  space — viz.,  angular  diameter  in  all 
directions — are  given  immediately.  But  this  does  not  give 
size,  unless  distance,  or  the  tJiird  dimension,  is  also 
known.  Now,  this  third  dimension  is  not  given  in  sense, 
but  is  a.  Judgment.  The  direct  gifts  of  sight  are  light,  its 
intensity,  its  color,  and  its  direction,  and  therefore  also  out- 
line form.  But  size,  distance,  and  solid  form  are  judg- 
ments based  on  these  gifts.  Moreover,  size  and  distance 
are  closely  correlated,  so  that  a  mistake  in  one  will  cause 
a  corresponding  mistake  in  the  other. 

Distance. — We  judge  of  distance  by  the  various 
forms  of  perspective  already  explained.  Being  a  judg- 
ment, we  are  liable  to  error.  We  often  say  "our  senses 
deceive  us."  Not  so.  We  make  false  judgments  on  true 
reports  of  the  senses. 

Size. — The  size  of  an  object  is  judged  by  lis  angular 
diameter,  or  size  of  its  retinal   image,  multiplied  by  its 


Fig.  94. 


estimated  distance.  For  example,  in  Fig.  94  the  reti- 
nal image  a  may  be  made  by  A  or  A'  or  A",  and  the  ap- 
parent  size  of  the  spatial  correspondent  will   vary  ac- 


154 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


cordingly.  If  we  imagine  it  at  A,  it  will  look  the  size 
of  A,  but  if  we  imagine  it  at  the  distance  ^4",  it  will  seem 
to  be  four  times  as  large.  If  its  real  size  and  place  is  A", 
and  we  imagine  it  to  be  at  A,  it  will  seem  many  times 
too  small.  If,  on  the  other  hand,  its  real  size  and  dis- 
tance is  represented  asyi  and  we  imagine  it  at  A",  it  will 
look  many  times  too  large.  For  example,  if  we  hold  up 
a  finger  before  one  eye  (the  other  being  shut),  very 
near  to  the  eye,  say  an  inch,  its  image  completely  covers 
a  large  building  one  hundred  yards  distant.  Now,  if  we 
imagined  the  finger  one  hundred  yards  distant,  it  would 
look  as  large  as  the  building. 

The  fact  of  dependence  of  apparent  size  on  estimated 
distance  is  well  shown  in  the  case  of  the  sun  and  the  moon. 
We  are  accustomed  to  estimate  the  distance  of  terrestrial 
objects,  but  have  no  means  of  judging  of  the  distance  of 
celestial  objects.  Therefore  different  persons  will  differ 
in  the  most  extraordinary  way  about  the  apparent  size  of 
the  sun  or  the  moon.  Some  will  say  that  they  look  about 
the  size  of  a  saucer,  others  the  size  of  a  dinner  plate, 
and  others  the  size  of  the  head  of  a  barrel.  There  are 
some  extreme  cases  of  persons  who  say  they  look  about 
the  size  of  an  orange,  and  others  as  big  as  a  cart  wheel. 

The  mathematical  relation  between  apparent  size  and 
estimated  distance  is  well  shown  by  spectral  images. 
Look  at  the  setting  sun  steadily  for  a  moment.  The 
image  of  the  sun  is  branded  on  the  retina  so  strongly 
that  the  brand  remains  for  some  time.  Now,  every 
change  in  the  retina,  whether  it  be  image  or  shadow 
or  brand,  is  seen  as  something  in  the  field  of  view. 
With  the  sun  brand  still  on  the  retina,  look  where  we 
will — on  the  wall,  on  the  floor,  on  the  sky — we  see  a  spec- 
tral image  of  the  sun.  Now  as  to  the  size.  Look  on  a 
sheet  of  paper  two  feet  off;  the  image  cast  on  the  sheet 
is  about  a  quarter  of  an  inch  in  diameter.     Look  at  the 


SENSE   ORGANS.  I  55 

wall  twenty  feet  off  ;  the  image  is  a  little  more  than  two 
inches  in  diameter.  Look  at  a  building  one  hundred 
feet  off;  the  image  is  about  ten  inches  in  diameter. 

Illustrations  meet  us  on  every  side.  In  a  fog  objects 
look  large,  because,  being  dim,  they  are  supposed  far- 
ther off  than  they  really  are.  In  the  exceptionally  clear 
atmosphere  of  Colorado  or  Nevada  objects  at  first  seem 
smaller  because  they  seem  nearer  than  they  are,  and 
they  seem  nearer  because  they  are  seen  so  plainly. 

Form. — Outline  form  is  a  combination  of  directions 
of  radiants,  and  is  therefore  seen  immediately.  We  are 
not  deceived.  But  solid  form  is  always  a  judgment.  We 
judge  sometimes  by  binocular  perspective,  sometimes  by 
shading  produced  by  light.  We  may  be  deceived  by 
skillful  shading  of  a  picture,  as  in  scene  painting. 

Gradations  of  Judgments. — There  are  all  degrees 
of  complexity  of  judgments  from  simple  gifts  of  sight 
on  the  one  hand  to  the  most  complex  intellectual  judg- 
ments on  the  other,  i.  Light,  its  intensity,  color,  and 
direction.  These  are  direct  gifts,  are  ultimate  facts,  and 
therefore  incapable  of  analysis.  2.  Then  come  outline 
form  and  surface  contents.  These  are  given  immediately, 
and  therefore  are  not  liable  to  deception,  but  are  capable 
of  analysis  into  simpler  elements — viz.,  a  combination  of 
directions.  3.  Next  comes  solid  form,  which  is  a  judg- 
ment, based  partly  on  binocular  perspective  and  partly 
on  the  shading  of  light.  Here,  for  the  first  time,  we  are 
liable  to  deception.  4.  Then  come  the  complex  judg- 
ments of  relative  distance  and  size  of  objects  in  an  ex- 
tensive landscape.  All  of  these  judgments  are  so  rapid 
that  they  are  usually  not  recognized  as  judgments  at  all. 
I  therefore  call  them  z7V«a/ judgments.  5.  These  pass 
by  insensible  gradations  to  the  simpler  intellectual  judg- 
ments, and  these,  in  their  turn,  into  the  most  complex 
process  of  thought-work. 


156   PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 

SECTION    VI. 
Comparative  Physiology  and  Morphology  of  the  Eye. 

VERTEBRATES. 

Mammals. — The  structure  of  the  eye  and  the  physi- 
ology of  vision  in  all  mammals  and,  indeed,  in  all  verte- 
brates is  substantially  the  same  as  that  already  given 
for  man,  yet  there  are  some  points  of  difference  worthy 
of  note. 

Color. — The  iris  is,  we  have  seen,  a  continuation  of 
the  choroid  coat.  Normally  and  most  usually,  there- 
fore, it  has  the  dark,  chocolate-brown  color  characteris- 
tic of  the  pigment  of  that  coat.  Doubtless  this  is  the 
original  and  normal  color  of  the  human  eye.  The  blue 
and  gray  are  the  result  of  peculiar  structure,  together 
with  a  deficiency  in  pigment.  Nearly  all  mammals  have 
the  normal  brown  color.  In  the  cat  tribe,  however,  as 
is  well  known,  it  is  brilliant  yellow. 

Pupil. — The  form  of  the  pupil  is  usually  round,  as  in 
man,  but  in  the  two  most  highly  specialized  and  differ- 
entiated orders — the  cat  tribe  on  the  one  hand,  and  the 
grazing  animals  on  the  other — the  pupil  is  greatly  elon- 
gated, vertically  in  the  former  and  horizontally  in  the 
latter.  The  vertical  elongation  is  probably  connected 
with  the  habit  of  springing  on  its  prey  ;  the  horizontal 
elongation,  certainly  with  wide  horizontal  view,  neces- 
sary in  grazing.  This  shape  of  the  pupil,  combmed  with 
the  prominence  of  the  eyes  and  their  position  on  the 
margin  of  a  broad  front,  makes  the  view  of  these  ani- 
mals sweep  the  whole  horizon  without  turning  the  head 
or  even  the  eyes. 

Tapetum. — In  many  mammals,  especially  those  of  noc- 
turnal habits,  such  as  the  cat  tribe  and  ruminants,  there 
is  found  at  the  bottom  of  the  retinal  concave  a  large 


SENSE   ORGANS. 


157 


patch,  which  has  a  bright,  iridescent  metallic  luster.  It 
is  called  the  tapetum.  It  is  a  modification  of  the  cho7-oid 
coat  for  the  purpose  of  reflection  of  light.  The  use  of 
it  is  not  well  understood,  but  it  is  believed  to  double  the 
impression  of  feeble  light  by  making  it  pass  twice  through 
the  retina.  It  is  this  that  causes  the  shining  of  the  eyes 
in  the  dark,  if  a  bright  light  is  present. 

Fovea. — There  is  in  all  mammals  a  central  area,  which 
is  a  little  more  sensitive  ;  but  a  true  fovea,  with  its  three 
characteristics  (explained  on  page  121),  is  not  found  in 
any  mammal  below  man,  except  the  anthropoid  apes. 

Birds. — The  iris  in  birds  is  very  various  in  color, 
most  commonly  the  normal  brown,  but  sometimes  yel- 
low, as  in  birds  of  prey,  sometimes  scarlet-red  (summer 
duck),  and  ?,om.G.i\mes porcelain-w/iite  (white-eyed  vireo). 

Sclerotic  Bones. — In  all  birds  and  many  reptiles 
we  find  a  series  of  bony  plates  in   the  front  part  of  the 

sclerotic   and  radiating 

•5  ^  from  the  margin  of  the 

iris.     These  are  beveled 

on  the  margins,  and  fit 


Fig.  95. — Eye  of  an  owl :  on,  optic 
nerve ;   c,  cornea ;   sb,  sclerotic  bones. 


Fig.  g6. — Sclerotic  bones  sepa- 
rated and  viewed  in  perspective. 


together  in  such  wise  as  to  slide  a  little  over  one  an- 
other. By  appropriate  muscles  these  may  be  made  to 
squeeze  the  ball  so  as  to  adapt  it  to  clear  vision  of  very 
near  objects  (Figs.  95  and  96). 

Nictitating  Membrane. — Birds  have  in  the  inner 
corner  of  the  eye  a   fold  of   the  conjunctiva  which   may 


158 


PHYSIOLOGY   AND    MORPHOLOGY    OF    ANIMALS. 


be  drawn  upward  over  the  eye,  wiping  it  and  protecting  it 
from  injury  without  entirely  excluding  the  light,  for  it  is 
semitransparent  (Fig.  97).  A  remnant  of  this  membrane, 
in  useless  condition,  is  found  even  in  man. 

Fovea. — Birds  not  only  have  a  fovea,  but  in  some  there 
are  ^ wo  in  each  eye.  The  most  distinct  of  these  is  in  the 
axis  of  the  eye,  and  therefore  at  the  bottom  of  the  retinal 
concave.  Now  since  the  optic  axes  are  not  parallel,  as 
in  man,  but  are  widely  divergent  (Fig.  too,  page  162), 
the  side  of  the  head  must  be  turned  toward  an  object  in 


Fig.  97. — Eye  of  a  bird  showing  (nm)  the  nictitating  membrane. 


order  that  its  image  shall  fall  on  this  fovea.  We  will 
speak  of  this  again  under  binocular  vision  in  vertebrates. 
Reptiles. — These  are  in  many  ways  similar  to  birds. 
The  sclerotic  bones  are  found  in  lizards  and  turtles  (Fig. 
98),  though  not  m  crocodiles  and  snakes.  In  some  rep- 
tiles— e.g.,  in  snakes — the  lids  are  absent.  The  dry, 
horny  epidermis  passes  directly  over  the  cornea  of  the 
eye,  and  in  skin-shedding  comes  off  with  the  rest  of  the 
epiderm.  Also  some  lizards — e.  g.,  chameleon  and 
phrynosoma— have  a  distinct  fovea. 


SENSE   ORGANS. 


159 


Fig.  98. ^Lizard's  eye  show- 
ing' the  sclerotic  bones. 
(After  Wiedersheim. ) 


Fishes. — In  these  the  lids  are  wanting,  the  eyes  be- 
ing kept  moist  by  the  water.  The  lens  of  fishes  is  very 
peculiar.  It  is  perfectly  spherical  and  much  denser  than 
in  land  animals.  Both  of  these 
qualities  give  greater  refractive 
power.  This  is  necessary  on  ac- 
count of  the  medium  in  which 
they  live,  for  the  refractive  pow- 
er of  the  eye  is  the  difference 
between  that  of  the  medium  and 
of  the  lenses.  This  is  well  illus- 
trated in  the  case  of  the  diver. 
Even  in  the  most  transparent 
water  vision  is  very  imperfect  if 
the  eye  is  immersed.  If  the  diver 
wishes  to  see  distinctly  under  water  he  must  supplement 
the  refractive  power  of  the  eyes  by  strong  double 
convex  lenses,  or  else  by  double  concave  air  spectacles. 
Such  spectacles  may  be  easily  extemporized  by  putting 
two  watch  glasses  back  to  back  and  cementing  imper- 
meable paper  about  the  margins.  It  is  evident  that 
these  would  act  precisely  like  two  convex  water-lenses 
in  air. 

The  ciliary  muscles  are  wanting  in  fishes.  They  first 
appear  in  amphibians — i.  e.,  in  the  lowest  land  verte- 
brates. Fishes,  therefore,  can  not  accommodate  the  eyes 
for  various  distances  by  changing  the  form  of  the  lens, 
for  it  is  already  spherical.  Their  eyes  are  passively  ad- 
justed for  near  objects.  They  probably  accommodate 
for  distant  objects  by  drawing  the  lens  back  nearer  to 
the  retina. 

Binocular  Vision  in  Vertebrates. — There  are  three 
points  of  structure  which  throw  light  on  this  subject — 
viz.,   (i)  the  optic  chiasm,  (2)   the   position  of  the  optiC 
axis,  and  (3)  the  fovea. 
12 


l6o  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Chiasm. — There  is  great  diversity  in  the  mode  of 
crossing  of  the  optic  nerves.  In  fishes  they  cross  bod- 
ily (Fig.  99,  a),  or  else  one  pierces 
the  other  (<^).  In  reptiles  they  form 
a  kind  of  basket-work  {c  and  d). 
Thus  far  there  is  a  complete  cross- 
ing of  fibers  in  a  more  or  less  com- 
plex way.  Each  side  of  the  brain 
controls  the  opposite  eye.  But  in 
birds,  and  especially  in  mammals, 
half  of  the  fibers  cross  and  half  do 
not  {e),  as  shown  more  fully  in  Fig. 
93,  page  148.  By  this  arrange- 
ment each  side  of  the  brain  sup- 
plies both  eyes,  and  each  eye  is 
controlled  by  both  sides  of  the 
brain;  and  therefore  the  two  eyes 
co-operate  as  one  instrument.  This 
arrangement  is  necessary  to  bin- 
ocular vision.  This,  therefore,  is 
the  only  true  chiasm.  It  is  prob- 
able, therefore,  that  no  animals 
below  birds  have  binocular  vi- 
sion. This  is  confirmed  by  the  po- 
sition of  the  eyes,  which  is  our 
next  point. 

Position  of  the  Eyes. — The  position  of  the  axes  of 
the  eyes  has  an  evident  relation  to  binocular  vision.  In 
man  the  two  eyes  are  directly  in  front,  with  the  axes 
parallel  in  a  passive  state.  From  this  state  of  parallelism 
they  may  be  easily  converged  on  a  near  object.  They 
are  therefore  in  the  best  possible  position  for  binocular 
vision.  The  same  is  true,  and  perhaps  in  equal  degree, 
in  apes.  But  below  this  the  eyes  are  wider  and  wider 
apart,  and  set  more  and  more  on  the   side  of  the  head. 


Fig.  99. — Different  modes 
of  crossing  of  the  optic 
nerves  :  a  and  b,  fishes  ; 
c  and  d,  reptiles ;  e, 
mammals. 


SENSE   ORGANS.  l6l 

The  difficulty  of  converging  on  a  near  point  becomes 
greater;  the  common  field  of  view  is  more  restricted, 
until  in  fishes  the  eyes  are  completely  on  the  side  of 
the  head  ;  the  optic  axes  diverge  one  hundred  and  eighty 
degrees;  convergence  on  a  point  is  impossible  ;  each  eye 
has  its  own  field  of  view,  which  do  not  overlap  to  make 
a  common  field,  and  therefore  they  can  not  have  binocu- 
lar vision. 

All  mammals  (except  perhaps  whales)  probably  en- 
joy binocular  vision  in  various  degrees  of  perfection. 
Birds  also  probably  are  similarly  endowed  (although 
their  eyes  are  so  widely  divergent),  but  this  is  by  virtue 
of  a  peculiar  structure,  to  be  spoken  of  under  the  next 
head. 

Fovea. — This  is  not  only  the  most  sensitive  spot  of 
the  retina,  but  it  is  the  center  about  which  the  corre- 
sponding points  of  the  two  retinae  are  symmetrically 
arranged.  It  is  undoubtedly  necessary  for  binocular 
vision  in  its  highest  perfection.  Now,  this  pitlike  spot  is 
found  among  mammals  only  in  man  and  the  anthropoid 
apes.  Mammals  generally  have  indeed  a  central  area 
(which  may  become  a  tapetum),  about  the  center  of  which 
corresponding  points  are  symmetrically  arranged,  but 
no  true  fovea.  It  is  probable  that  in  them  the  advan- 
tages of  accurate  observation  of  a  single  thing  is  sacri- 
ficed to  the  much  greater  advantages  of  somewhat  dis- 
tinct vision  over  a  wide  field. 

In  birds  the  fovea  again  appears,  and  yet  their  optic 
axes  are  so  widely  divergent  as  to  make  it  impossible  to 
converge  these  axes  on  a  point  (see  Fig.  loo).  Never- 
theless, birds  seem  to  have  binocular  vision,  but  this  is 
by  virtue  of  atwther  fovea.  In  other  words,  among  all 
animals  birds  are  peculiar  in  having  two  foi^ece  in  each  eye, 
one  monocular  and  the  other  binocular.  The  monocular 
ones,  aa\  are  axial  and  are  the  more  distinct ;  the  binoc- 


l62  PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


ular  ones,  b  b',  are  far   removed   from  the  axis   on  the 
temporal  side,  and  so  situated  that  lines  drawn  through 

them  and  through  the  pu- 
pils are  parallel.  These 
axes  can  be  converged  on 
a  given  point,  and  doubt- 
y  less  corresponding  points 
are  symmetrically  ar- 
ranged about  them  and 
not  about  the  other.  The 
central  or  monocular 
fovea  is  the  most  distinct, 
and  therefore  the  monoc- 
ular vision  is  better  than 
the  binocular.  This  is  the 
reason  why  birds — for  ex- 

FiG.  ioo.-Secti,^rjbird's  head  (after  ^^ple,  the  domestic  fowl 

Slonaker):    T' T',   monocular  visual  — in     looking    attentively 
lines;   vv\   binocular   visual   lines; 

a  a',  d^',  central  and  temporal  foveae  turn    the    head    and    look 

respectively.  ^j^^  ^^^  ^^^ 

Below  birds,  except  in  some  lizards,  nothing  like  a 
distinct  fovea  is  found. 

It  seems  certain,  therefore,  that  binocular  vision  in 
its  most  perfect  form  is  found  only  in  man  and  the  higher 
apes,  and  thence  becomes  gradually  less  and  less  per- 
fect until  it  disappears  entirely  in .  the  lowest  verte- 
brates. It  is  almost  needless  to  add  that  it  is  not  found 
at  all  in  invertebrates. 


INVERTEBRATES. 

In  all  that  follows  we  are  compelled  to  be  very  brief, 
touching  only  most  salient  points.  Some  of  these  points 
will  come  up  again  under  Evolution  of  the  Eye. 

We  pass  over  the  arthropods,  because  in  most  of 
them  the  structure  of  the  eye  is  so  different  from  what 


SENSE   ORGANS. 


163 


we  have  described  in  vertebrates  that  no  comparison 
can  be  instituted.  In  them  we  find  a  different  ^/«^  of 
instrument  and  not  a  mere  modification  and  simplifica- 
tion of  that  already  studied.     We  shall  come  back   to 

these  after  completing 
the  comparison  in  the 
case  of  other  inverte- 
brates. 

Mollusca :  Ceph- 
alopods. — The  higher 
cephalopods,  such  as 
the  squid  and  cuttlefish, 
have  large  eyes  and  by 
far  the  most  perfect 
below  vertebrates  (Fig. 
roi).  Their  instrumen- 
tal structure  is  substan- 
tially like  that  of  ver- 


FlG.  101. — Nervous  system  of  an  argonaut, 
showing  the  eyes  :  eg-,  cephalic  ganglion  ; 
og,  the  optic  ganglion  ;  mg,  brg,  vg,  the 
ganglia  of  the  mantle,  the  branchise,  and 
the  viscera,  respectively  ;  E,  eye.  (After 
Cuvier. ) 


Fig.  102. — The  eye 
of  a  snail  on  the 
end  of  the  ten- 
tacle, magnified. 


tebrates  and  is  fully  as  perfect  as  that  of  a  fish.  There 
are,  indeed,  some  very  significant  differences,  especially 
in  the  retina,  but  these  will  come  in  our  discussion  of  the 
evolution  of  the  eye. 

In   the  gastropods  the  lois  is   wanting,   the   vitreous 


164  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

humor  being  the  only  refractive  medium.  The  eyes  of 
these  are,  of  course,  situated  about  the  head,  and  often, 
as  in  snails,  on  the  ends  or  at  the  base  of  the  tentacles 
or  so-called  horns  (Fig.  102). 

The  acephala,  or  bivalves,  as  the  name  indicates,  are 
without  distinct  head,  and  the  eyes  or  eye-spots  are 
strung  along  on  the  margins  of  the  mantle,  as  in  pecten. 
But  in  many  lower  acephala  and  in  echinodermata  and 
coelenterata  the  lens  or  any  kind  of  refracting  image- 
making  instrument  disappears,  and  the  eye  is  reduced  to 
a  deposit  ol  pigment  to  absorb  the  light  and  a  specialized 
nerve  to  respond  to  light.  These  are  called  eye-spots. 
They  differ  from  true  eyes  in  not  forming  an  image. 
They  perceive  light,  but  not  objects.  They  have  the  spe- 
cialized nerve,  but  not  the  image-making  instrument. 

Arthropods. — We  passed  over  these  because  out  of 
the  direct  line  of  evolution.     We  now  return. 

Many  arthropods — for  example,  the  spider — have 
eyes  on  the  same  plan  as  other  invertebrates,  but  usu- 
ally very  small.  But  the  most  characteristic  eye  of  ar- 
thropods is  the  compound  eye  of 
insects  and  crustaceans.  It  is  ne- 
cessary, however,  before  describing 
these,  to  say  something  of  the  si>?i- 
ple  eye  of  arthropods. 

Simple  Eye. — If  a  spider  be  ex- 
amined with  a  hand  lens,  a  number 
''^  of  brilliant,  gemlike  spots  are  seen 

Fig.  103. — Eyeof  a  spider  :  ° 

z,  lens;  F,  vitreous  hu-  on  the  front  part  of  the  cephalo- 
TaV^^/opdc^rier^e'''"  thorax.  They  are  usually  in  groups 
of  four,  six,  and  eight  on  each  side. 
These  are  the  eyes.  Though  small,  they  are  somewhat 
perfect  for  invertebrate  eyes,  for  we  find  a  cornea,  c 
(Fig.  103),  a  lens,  Z,  a  vitreous  humor,  F,  a  retina,  r, 
and  an  optic  nerve,  on. 


SENSE   ORGANS. 


165 


Compound  Eye. — The  compound  eye  of  insects  and 
crustaceans  is  very  different.     If  we  examine  the  head 
of  any  insect,  such  as  a  fly,  a  dragon  fly,  a  butterfly,  or 
a   beetle,    we   find   that    it    consists 
largely  of  two  great  hemispherical 
masses,    often    of  brilliant  metallic 
luster,  green,  or  purple,  or  yellow. 
These  are  the  two  compound  eyes 
(Fig.  104).     If  their  surface  be  ex- 
amined with  a  hand  lens,  or,  better, 
if  the  outer  transparent  corneal  por- 
tion be  removed  and  placed  under     Fig.  104.— Anterior  part 

.  .  of  a  dragon  fly,  show- 

a  microscope,  we  see  that  it  consists         ing  the  compound  eyes. 

of  thousands  (twenty-eight  thou- 
sand in  the  dragon  fly)  of  transparent  hexagonal  plates 
nicely  fitted  together  (Fig.  105).  Each  plate  covers  a 
hexagonal  prism,  which  runs  back  to  abut  against  the 
convex  surface  of  the  optic  ganglion,  which  acts  as  the 
retina  and  connects  in  its  turn  through  the  optic  nerve 
with  the  cephalic  ganglion.  Each  tube 
is  lined  with  pigment,  which  may  be 
likened  to  a  choroid,  and  filled  with  a 
transparent  substance,  which  may  be 
likened  to  the  vitreous  humor.  The 
whole  is  covered  with  a  hexagonal  cor- 
neal plate,  which  is  thickened  into  a 
kind  of  lens  over  each  prism.  One 
prismatic  element  is  called  an  ommatidium  (Fig.  106). 

Now  see  in  a  general  way  (for  it  is  not  well  under- 
stood) how  vision  is  accomplished  by  this  instrument. 
Remember,  the  condition  of  distinct  image  is  that  each 
radiant  should  impress  its  own  focal  point  on  the  ret- 
ina. Rays  from  several  points  must  not  mix  (page 
104).  Now  if  an  object,  A  B  (Fig.  106),  be  placed  before 
such  an  eye,  the   central  ray   from  each  point,  ABC, 


.^^Ekf^^^ 


W0Bi 


Fig.  105. — A  portion 
of  corneal  surface 
of  the  compound 
eye  magnified. 


1 66  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Fig.    io6.  —  Diagram    section    through 
compound  eye  :  C,  cephalic  ganghon. 


passes  down  the  corresponding  tube  and  impresses  its 
own  point  on  the  retina,  and  thus  forms  the  image.     Rays 

passing  into  other  tubes 
strike  the  pigmented 
sides  and  are  quenched, 
and  thus  mixing  is  pre- 
vented, but  with  great 
loss  of  light. 

Comparison  with  Si/n- 
ple  Eyes.  —  Comparing 
now  with  normal  eyes 
of  invertebrates,  we  see 
great  differences  in  sev- 
eral respects,  i.  In  or- 
dinary eyes  distinctness 
is  reached  by  bringing 
all  the  rays  from  each 
radiant  to  a  single  focal 
point  on  the  retina.  In  this  method,  on  the  contrary, 
the  same  result  is  secured,  but  with  great  loss  of  light, 
by  allowing  only  the  central  rays  from  each  radiant  to 
reach  and  impress  the  retina.  2.  In  all  other  eyes,  ver- 
tebrate or  invertebrate,  the  image  is  inverted;  in  this, 
on  the  contrary,  it  is  erect  (Fig.  106).  Nevertheless,  in 
this  case  also,  by  the  law  of  direction,  the  object  is  seen 
erect.  The  reason  is  that  in  all  other  eyes  the  recipient 
surface  is  concave,  and  therefore  reinverts  the  image  in 
the  act  of  external  reference,  while  in  this  it  is  convex, 
and  does  not  reinvert  the  image.  3.  In  vertebrate  eyes 
a  wide  field  is  got  by  free  motion  of  the  eye  in  its 
socket;  but  in  compound  eyes  it  is  got  by  the  sphericity 
of  the  large  surface.  In  Crustacea,  where  the  sphericity 
is  less  great,  the  eye  is  placed  on  the  end  of  a  movable 
stalk.  It  is  probable  that  the  sight  of  the  compound 
eye  is  very  imperfect  except  at  short  distance. 


SENSE   ORGANS.  1 6/ 

Origin  of  the  Compound  Eye. — The  spider  has  many 
very  small  simple  eyes  in  two  groups,  one  on  each 
side  of  the  head.  Now  imagine  the  number  greatly  in- 
creased, the  size  correspondingly  diminished,  and  then 
the  whole  group  crowded  together  until  by  mutual 
pressure  they  are  squeezed  and  elongated  into  pris- 
matic tubes,  and  we  have  a  general  idea  of  the  prob- 
able process  of  change. 


EVOLUTION    OF    THE    EYE. 

The  exquisite  beauty  of  the  mechanism  of  the  eye 
makes  its  evolution  extremely  interestmg  ;  but  hereto- 
fore it  has  seemed  an  insoluble  mystery.  Recently,  how- 
ever, much  light  has  been  thrown  on  the  subject. 

I.  Invertebrate  Eye. — General  sensibility  to  light 
is  coextensive  with  life  itself.  But  it  is  a  law  in  biology 
that  any  useful  function  will  be  gradually  separated 
from  other  functions,  localized  in  an  organ,  and  then  im- 
proved indefinitely.  How  did  a  light-perceiving  organ 
begin  ?  It  probably  began  to  be  formed  under  the 
stimulus  of  light  itself,  as  follows  : 

(i)  On  the  exposed  epithelial  surface  certain  spots 
became  pigmented,  and  thus  more  absorbent  of  light ; 
the  nerves  to  these  spots  became  specialized  to  respond 
to  the  light  ;  the  epithelial  cells  of  these  spots  became 
slightly  modified  by  elongation  into  rodlike  form;  and 
already  we  have  an  eye-spot,  the  simplest  beginnings  of 
an  eye.  Why  this  effect  should  occur  only  ///  spots  we 
know  not,  any  more  than  we  know  why  freckles  should 
come  in  spots.  Such  eye-spots  may  occur  anywhere  in 
exposed  surfaces,  but  more  commonly  on  the  most 
sensitive  part,  viz.,  the  head,  when  there  is  a  head. 
This  first  step  is  found  in  very  many  lowest  animals, 
especially  in  lowest  mollusks  (Fig.  107,  a). 


l68  PHYSIOLOGY   AND    MORPHOLOGY   OF  ANIMALS. 

(2)  The  next  step  is  a  slight  saucerhke  depression  of 
the  pigmented  spot  with  an  increased  pigmentation  and 
elongation  of  the  cells.  This  step  is  found  in  the  sword 
shell  {Solen),  in  which  the  eye  spots  are  strung  all  along 
the  edge  of  the  mantle  as  the  only  exposed  part,  for 
these  animals  are  headless  (Fig.  107,  b). 

(3)  In  the  next  step  the  depression  becomes  deep, 
cuplike.  Evidently  here  there  is  a  stronger  impression 
of  light  by  reverberation  in  the  hollow  and  consequent- 


FiG.  107. — Diagram  representing-  the  different  stages  in  the  evolution  of  the 
invertebrate  eye  :  a  b  c,  eye-spots,  no  image  ;  d,  pin-hole  image  ;  e,  sim- 
ple lens  image  ;  /",  compound  lens  image  ;  r,  retina  ;  on,  optic  nerve  ; 
V,  vitreous  humor  ;  /,  lens  ;  cor,  cornea. 


ly  a  greater  specialization  of  cells  for  response.  This 
step  is  found  in  the  limpet  or  Patella,  and  the  organ  is 
situated  in  the  head,  for  this  is  a  gastropod.  Already 
we  begin  to  see  in  the  pigmentary  layer  a  choroid  and 
in  the  elongated  rodlike  cells  a  bacillary  layer  of  a  retina 
(Fig.  107,  c). 

Thus  far  we  have  only  eye-spots,  not  an  eye  proper  ; 
only  a  specialized  layer  of  nerve  terminals,  not  an  im- 


SENSE   ORGANS.  169 

age-making  instrument ;  an  organ  perceiving  light,  but 
not  yet  seeing  objects. 

(4)  In  the  next  step,  which  is  found  in  the  nautilus,  the 
cup-shaped  depression  is  closed  in  above  until  it  becomes 
a  hollow  vesicle  with  only  a  pin-hole  opening  atop.  Now 
for  the  first  time  we  have  an  image,  an  inverted  image, 
on  what  is  now  plainly  a  retina  (Fig.  107,  d^.  Now  for 
the  first  time  there  is  a  perception  not  only  of  light, 
but  also  objects.  In  a  word,  we  have  a  true  eye.  But 
the  sight  of  objects  is  still  imperfect,  for  it  is  only  a 
pin-hole  image. 

(5)  In  the  next  step  the  pin-hole  opening  closes,  but 
the  point  of  closure  remains  transparent  as  a  cornea, 
and  the  cavity  or  vesicle  thus  formed  (optic  vesicle)  is 
filled  by  secretion  with  a  transparent  refractive  sub- 
stance which  may  be  regarded  as  a  vitreous  humor. 
We  have  now  for  the  first  time  a  lens  image,  but  yet 
only  a  simple  lens  image  (Fig.  107,  e).  This  is  the  case 
in  the  snail  and  many  other  gastropods. 

(6)  Finally  in  the  squid  the  last  stage  in  this  strange, 
eventful  history  is  found.  In  these  there  is  a  cutic- 
ular  ingrowth  from  the  corneal  surface  which  finally 
separates  as  a  crystalline  lens  (Fig.  107,/),  and  thus  we 
have  a  compound  lens  image. 

That  these  are  really  the  steps  of  evolution  of  the 
eye  is  proved  by  the  fact  that  in  embryonic  develop- 
ment the  squid's  eye  passes  through  all  these  stages.  It 
is  first  seen  as  a  dark  spot,  then  as  a  saucerlike  de- 
pression, then  as  a  cup-shaped  depression,  then  as  a 
hollow  cavity  with  a  pin-hole  aperture;  then  the  aper- 
ture closes  and  the  vesicle  fills,  and,  lastly,  the  crys- 
talline lens  is  formed  by  cuticular  ingrowth  from  the 
cornea.  This  is  the  most  perfect  eye  found  among 
invertebrates. 

In  the  invertebrate  eye  there  is  yet  no  chiasm  (Fig. 


I/O 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


loi,  page  163),  nor  is  there  any  fovea.  There  are  cer- 
tainly no  corresponding  points  of  the  two  retinae,  and 
therefore  no  binocular  vision  ;  also,  as  we  shall  see  pres- 
ently, no  blind  spot. 

2.  The  Vertebrate  Eye. — There  are  two  essential 
differences  between  the  invertebrate  and  the  vertebrate 
eye.  (i)  In  the  former  the  nerve  fibers  terminate  for- 
ward in  the  posterior  ends  of  the  rods  in  the  most 
natural  way,  as  in  the  case  of  nerve  terminals,  in  all  other 
sense-organs.  The  bacillary  layer  is  the  innermost 
layer  of  the  retina,  and  exposed  directly  to  the  action 
of  light.  In  the  vertebrate  eye,  on  the  contrary,  the 
bacillary  layer  is  the  outermost  layer  of  the  retina,  and 
therefore  the  fibers  have  to  go  forward  beyond  and 
turn  back  and  terminate  in  the  anterior  ends  of  the  rods. 
This  is  wholly  exceptional  not  only  among  eyes,  but 
among  special  sense-organs.  It  is  this  course  of  the 
fibers  which  makes  a  blind  spot,  and  therefore  the  in- 
vertebrate eye  can  not  have  a  blind  spot. 

(2)  In  invertebrates  the  whole  eye,  both  the  retina 
and  the  lenses,  is  made  by  infolding  of  an  external  epi- 
thelial surface.  In  vertebrates,  on  the  contrary,  the 
instrumental  part,  especially  the  crystalline  lens,  is  made 
in  this  way,  but  the  retinal  part  is  made,  as  embryonic 
development  shows,  from  the  brain,  by  an  outfolding  of 
the  cerebral  vesicle. 

The  steps  of  the  development  of  the  vertebrate  eye 
are  briefly  as  follows:  (i)  The  brain  is  developed  as 
three  vesicles.  The  anterior  one  is  the  thalamus  (Fig.  22, 
page  37),  which  is  the  basal  part  of  the  cerebrum,  and  we 
shall  call  this  the  cerebral  vesicle.  (2)  From  the  cerebral 
vesicle  by  outfolding  is  formed  on  each  side  the  optic  vesi- 
cles {OV,  Fig.  108,  A),  which  become  more  and  more  con- 
stricted off  until  they  are  connected  only  by  a  narrow 
neck,    which  becomes    the   optic    nerve    (Fig.    108,    B). 


SENSE    ORGANS. 


171 


(3)  Meanwhile  the  infolding  from  the  epidermal  sur- 
face has  formed  the  lens.  (4)  Then  the  optic  vesicle 
becomes  folded  back  upon  itself  like  a  double  nightcap, 
so  as  to  leave  a  large  space  between  it  and  the  lens. 
The  anterior  or  back-folded  layer  of  the  double  night- 
cap becomes  the  retina,  and  the  posterior  layer  the 
choroid  (Fig.  108,  C).     (5)  The  two  folds  come  in  con- 


FlG.  108. — Diagram  representing  different  stages  in  the  development  of  the 
vertebrate  eye  :  C  V,  cerebral  vesicle  ;  O  l',  optic  vesicle  ;  r,  retina  ;  c/i, 
choroid  ;  d,  bacillary  layer  ;  /,  fibrous  layer  ;  /,  lens. 

tact  and  the  vesicle  is  obliterated.  The  space  between 
the  concave  retina  and  the  lens  is  filled  and  forms  the 
vitreous  humor,  the  whole  becomes  encysted  by  the 
sclerotic,  and  the  eye  is  finished.  Now  observe  that 
the  cerebral  vesicle  and  the  optic  vesicle  are  lined 
with   epithelium.      When   this    is    folded    back   the  pos- 


172 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


terior  or  epithelial  layer  of  the  back-folded  nightcap 
becomes  the  bacillary  layer  of  the  retina,  while  the 
anterior  layer  of  the  same  is  the  fibrous  layer  of  the 
retina. 

The  difference  between  the  vertebrate  and  inverte- 
brate retina  is  great,  but  not  so  great  as  at  first  seems. 
In  both  cases  the  bacillary  layer  is  formed  of  epithelial 
cells — in  the  one  of  exterior  epithelium  of  skin,  in  the 
other  of  interior  epithelium  of  the  brain.  But  the  lining 
epithelium  of  the  brain  is  itself  an  infolded  portion  of 
the  epiderm  (Fig.  108,  A  and  B). 

Transition  from  Invertebrate  to  Vertebrate 
Eye. — If  vertebrates  came  from  some  form  of  inverte- 
brates, as  undoubtedly  they  did,  how  was  the  vertebrate 
eye  evolved  out  of  the  invertebrate  eye  ?  This  is  a  very 
difficult  question,  i.  Of  course,  the  vertebrate  type  of 
eye  must  have  branched  off  very  low  down  and  before 
the  invertebrate  type  was  fully  declared.  2.  Much  of  the 
difficulty  has  come  of  identifying  the  crystalline  lens  of 
the  vertebrate  eye  with  the  same  of  the  invertebrate 
eye.  On  the  contrary,  it  corresponds  to  the  whole  eye 
of  the  invertebrates.  It  is  formed  in  the  same  way, 
viz.,  by  infolding  of  the  epidermal  surface,  while  the 
lens  of  invertebrates  is  formed  by  cuticular  ingrowth 
from  the  corneal  surface.  The  retina  of  the  verte- 
brate eye  is  something  superadded  to  the  whole  eye  of 
invertebrates,  the  retinal  part  of  the  latter  having  been 
aborted  and  modified  to  form  the  back  part  of  the  ver- 
tebrate lens. 

Thus  much  seems  certain,  but  how  the  change  came 
about  is  obscure.  We  may  imagine  {a)  some  low  form 
of  invertebrate  with  very  imperfect  invertebrate  eye,  the 
infolded  epiderm  functioning  as  usual  as  retina,  but  this 
very  close  to  cephalic  ganglion.  The  light  stimulating 
the  cephalic  ganglion  might  well  provoke  the  formation 


SENSE   ORGANS. 


173 


of  anothe«r  and  better  retina.  This  functions  as  retina, 
while  the  whole  invertebrate  eye  is  transformed  into  a 
lens.  Or  (d)  some  low  form  may  have  had  a  pigmented 
spot  on  each  side  of  the  anterior  part  of  the  head.  In 
the  formation  of  the  brain  and  spinal  cord  by  infolding 
of  epiderm  these  spots  might  well  be  carried  into  the 
cerebral  vesicle  and  thence  into  the  optic  vesicle  and 
become  a  retina.  Meanwhile  the  lens  was  formed  by  in- 
folding, as  already  explained. 

Further  Evolution  of  the  Vertebrate  Eye. — 
However  this  may  be,  once  the  vertebrate  plan  is  estab- 
lished the  process  of  improvement  goes  on  again  stead- 
ily. In  fishes  the  position  of  eyes  on  the  side  of  the 
head  and  the  absence  of  true  chiasm  show  that  there 
are  as  yet  no  corresponding  points,  and  therefore  no 
binocular  vision.  The  ciliary  muscle  is  also  wanting, 
and  the  eye  can  not  be  accommodated  to  accurate 
vision  for  various  distances  in  the  same  way  as  in  land 
vertebrates.  The  eye  is  no  better  than  that  of  the 
squid. 

But  in  land  animals  the  lens  becomes  flattened  to 
double  conve-x  shape,  and  may  now  be  accommodated 
to  different  distances  by  action  of  the  ciliary  muscle. 
A  true  chiasm  is  not  formed,  and  therefore  binocu- 
lar vision  is  not  evolved  until  we  reach  birds.  Mean- 
while the  eyes  are  moved  more  and  more  to  position 
in  front,  with  increasing  capability  to  converge  on  a 
given  point ;  corresponding  points  are  established  in 
the  retinae;  binocular  vision  and  judgments  appertain- 
ing thereto  become  possible  and  more  and  more  per- 
fect. Finally,  there  is  added  a  fovea,  and  with  it  the 
ability  to  fix  undivided  attention  on  the  objects  looked 
at,  and  this,  in  its  turn,  is  at  least  one  necessary  con- 
dition of  the  evolution  of  the  higher  faculties  of  the 
mind. 


174    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

SECTION    VII. 
Sense  of  Hearing  and  i/s  Organ^  the  Ear. 

Sight  and  hearing  are  the  two  higher  senses.  In 
these  alone  the  impression  of  the  sensible  body  is  not  on 
the  specialized  nerve  directly,  but  indirectly  through  the 
vibrations  of  a  medium.  In  these  alone,  therefore,  in 
addition  to  the  specialized  nerve  and  in  front  of  it,  there 
is  a  mechanical  instrument  for  making  the  impression 
stronger  and  more  definite. 

The  eye  is  undoubtedly  the  most  refined  mechanism 
in  the  animal  body,  and  yet  its  structure  is  more  easily 
explained  than  that  of  the  ear.  The  structure  of  the  ear 
is  not  only  very  complex,  but  it  is  lodged  in  intricately 
winding  passages  in  the  interior  of  the  hardest  bone  in 
the  body.  In  these  passages  the  branches  of  the  eighth 
pair  of  nerves  are  distributed  and  specialized  to  respond 
to  vibrations  of  the  air. 

Structure  of  the  Human  Ear. — The  ear  consists 
of  three  general  parts — the  exterior,  the  middle,  and  the 
interior  ear.  The  first  two  are  air-filled,  the  third  is  en- 
tirely cut  off  from  the  air  and  is  water-filled.  The  first 
two  are  instrumental ;  the  third  alone  contains  the  spe- 
cialized nerve  (Fig.  109). 

The  exterior  ear  includes  all  that  is  visible  from  the 
outside — i.  e.,  as  far  as  the  membrane  of  the  drum.  It 
consists  of  the  conch  and  the  meatus.  The  conch 
collects  the  aerial  vibrations,  and  the  meatus  carries 
them  to  the  membrane  of  the  drum.  The  meatus  se- 
cretes a  kind  of  wax — ear  wax — which  by  accumu- 
lation may  cause  partial  deafness,  but  is  easily  re- 
moved. 

The  mid-ear  is  a  cavity  just  beyond  the  membrane 
of  the  drum.     It  is  about  one  third  of  an  inch  in  diame- 


SENSE   ORGANS. 


175 


ter  in  direction  at  right  angles  to  the  drumhead,  and 
three  quarters  of  an  inch  in  a  direction  up  and  down. 
It  is  connected  with  the  throat  by  a  slender  tube — the 
Eustachian  tube — and  is  therefore  air-filled.  The  closure 
of  this  tube  by  inflammation  of  the  throat  is  a  frequent 
cause  of  partial  deafness.  By  holding  the  nose  and 
blowing  hard,  air  may  be  forced  through  the  Eustachian 


\-,'" ' 


Fig.  109. — Simplified  diagram  representing  a  section  through  the  ear ;  the 
cochlea  is  supposed  to  be  unrolled  :  ma,  meatus  auditorius  ;  /,  tympanic 
membrane  ;  ni,  in,  st,  the  ossicles  ;  sc,  semicircular  canals  ;  c,  cochlea; 
scv,  set,  scala  vestibuli  and  scala  tympani ;  eu,  eustachian  tube.  The 
shaded  part  represents  bone.     ( From  Huxley. ) 


tube  into  the  drum,  and  cause  sensible  pressure  on  the 
membrane  of  the  drum. 

This  cavity  is  separated  from  the  outer  ear  by  the 
membrana  tympani,  and  from  the  inner  ear  by  a  bony 
wall,  in  which  are  two  openings  closed  with  membrane, 
viz.,  the  foramen  rotundum  and  foramen  ovale.  These 
membranes  act  as  counter-drumheads  to  the  membrane 
of  the  drum, 
13 


1^6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Ossicles. — Running  across  from  the  membrane  of 
the  drum  to  the  foramen  ovale  is  a  chain  of  three  little 
bones,  called   the  iiialleus  (hammer),    the   incus    (anvil), 


St.  in.  m. 

Fig.  iio. — Ossicles,  enlarged:  st,  stapes;  in,  incus;  ;«,  malleus. 

and  the  stapes  (stirrup).  These  are  articulated  together 
and  to  the  walls  of  the  cavity  in  a  somewhat  intricate 
way,  but  evidently  contrived  to  carry  vibrations  of  the 
drumhead  to  the  interior  ear,  for  the  malleus  is  attached 
to  the  drumhead,  the  stapes  to  the  membrane  of  the 
foramen  ovale,  and  the  incus  is  intermediate.  Sound 
vibrations  of  the  air  cause  corresponding  vibrations  of 
the  drumhead  (/),  and  these  are  carried  along  the  chain 
of  bones  and  shake  the  membrane  of  the  oval  opening, 
and  therefore  the  water  filling  the  inner  ear,  where  its 
further  effects  will  be  given  after  the  inner  ear  is  de- 
scribed. The  actual  shapes  of  these  bones  are  shown 
in  Fig.  1 10. 

Interior  Ear,  or  Labyrinth. — The  real  receptive 
part  of  the  ear  is  here.  All  other  parts  are  purely  in- 
strumental. It  is  called  the  labyrinth  because  of  the 
complex  winding  passages  in  which  the  branches  of  the 
auditory  nerve  are  distributed.  The  labyrinth  may  be 
best  described  under  two  heads,  viz.,  the  bony  labyrinth 
and  the  membranous  labyrinth.  The  bony  labyrinth 
consists  of  winding  cavities  in  the  solid  bone  ;  the  mem- 


SENSE   ORGANS. 


177 


branous  labj'rinth  is  a  membranous  apparatus  lodged  in 
these  cavities.  Each  of  these  two  parts  consists  of 
three  parts,  viz.,  the  vestibule,  the  semicircular  canals,  and 
the  cocJilea.  There  is  therefore  a  bony  vestibule,  semi- 
circular canals,  and  cochlea,  and  a  membranous  vesti- 
bule (vestibular  sac),  semicircular  canals,  and  cochlea. 
All  these  parts  in  the  membranous  apparatus  have  forms 
similar  to  the  bony  cavity  in  which  they  are  lodged,  but 
are  much  smaller,  so  that  there  is  considerable  space  be- 
tween the  true  receptive  membranous  part  and  the  cav- 


FlG.  III. — Outer  and  middle  ear  seen  in  section,  and  the  inner  as  a  cast  of 
the  bon)'  labyrinth  :  nia,  meatus  auditorius  ;  7nc,  mastoid  cells  ;  t,  tym- 
panic membrane  ;  ;«,  malleus  ;  /',  the  incus ;  z/,  vestibule  ;  c ,  cochlea  ; 
sc^  semicircular  canals  ;  aim,  auditory  nerve.     (  After  Cleland. ) 


ity  in  which  it  is  lodged.  This  space  is  filled  with  a 
watery  liquid  called  perilymph.  The  membranous  ap- 
paratus is  also  filled  with  a  liquid  called  endolymph.  In 
the  diagram  (Fig.  109)  the  dotted  parts  represent  the 
membranous  labyrinth. 


1^8    PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


Fig. 


-Section  of  the  cochlea  showing  the 
winding  cavity. 


Bony  Labyrinth. — The  vestibule  is  a  hollow  space 
about  the  size  of  a  wheat  grain.  It  is  called  the  vesti- 
bule because  it  is 
the  general  hall  into 
which  open  all  the 
winding  passages. 
It  is  separated  from 
the  tympanic  cavity 
by  a  wall  in  which 
is  the  membrane- 
closed  oval  open- 
ing. Out  of  this 
hall  go  out  and 
return  again  three 
slender  tubes — the 
semicircular  canals. 
Two  of  these  unite 
at  one  end  and  enter  by  a  common  opening,  so  that 
there  are  five  openings  into  the  vestibule  instead  of 
six.  At  one  end  of  each  canal  there  is  a  flasklike  en- 
largement. 

The  bony  cochlea  is  a  spiral  cavity  like  a  spiral  stair- 
way, winding  about  a  central  pillar  two  and  a  half 
times  and  growing  smaller  to  the  end  (Fig.  112).  The 
name  is  taken  from  its  resemblance  to  the  shell  of  a 
snail. 

Membranous  Labyrinth. — In  the  bony  labyrin- 
thine cavity  just  described  is  lodged  the  membranous 
parts  of  the  same  names.  The  membranous  vestibule, 
or  vestibular  sac,  within  the  cavity  of  the  bony  vestibule 
is  a  small  sac  from  which  go  and  return  the  three  mem- 
branous semicircular  canals,  each  with  its  flask-shaped 
enlargement  at  one  end,  called  the  ampulla.  The  audi- 
tory nerves  are  distributed  on  the  vestibular  sac  and  on 
the  ampulLne,  the  fibers  terminating  directly  on  the  inte- 


SENSE  ORGANS. 


179 


rior  surface  (Fig.  113).  In  the  vestibular  sac  and  at- 
tached to  hairlike  nerve  terminals  there  are  several  little 
sandlike  grains  of  carbonate  of  lime  (otoliths).  By  mo- 
tion of  the  endolymph  these  are  shaken  and  affect  the 
hairlike  nerve  terminals,  which  thus  become  delicate 
perceivers  of  the  slightest  movements  caused  by  vibra- 
tion. Again,  in  the  ampullae  the  nerve  fibers  terminate 
in  little  stififish  hairs  projecting  from  the  walls  toward 
the  center  like  the  hairs  in  a  mule's  ears  (Fig.  114). 
Vibratory  shakings  of  the  endolymph,  passing  up  from 
the  vestibular  sac  through  the  ampullae  into  the  semi- 


's c 


Fig.  113. — vs,  vestibule  sac;  sc,  membranous  semicircular  canals;  a/fi,  am- 
pulla ;  n,  nerve  ;  J,  sacculus. 


circular  canals  and  back  to  the  vestibular  sac,  set  these 
hairs  trembling,  and  hence  they  also  become  delicate 
perceivers  of  slight  vibration. 

Membranous  Cochlea. — We  have  compared  the 
bony  cochlea  to  a  hollow  stairway  winding  about  a 
central  pillar  ;  now  the  membranous  cochlea  may  be 
compared  to  the  s/air  in   this  stairway,  running  across 


l8o   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


from  a  bony  ledge  on  the  pillar  to  the  outer  wall  of  the 
stairway,  and  dividing  it  into  two  semicylindrical  spiral 
hollows,  one  above  and  one  below,  but  not  quite  reach- 
ing the  extreme  end  ;  so  that  vibration  might  run  spirally 
around  in  the  upper  way,  over  at  the  top,  and  down 
spirally  by  the  lower  way.  These  two  ways  are  called 
the  one  scalavestibuli,  because  it  opens  into  the  cavity  of 
the  bony  vestibule,  and  the  other  the  scala  tympani,  be- 
cause it  abuts  against  the  tympanum,  being  separated 
from  it  only  by  the  membrane  of  the  foramen  rotundum. 
The  whole  bony  cavity  of  the  cochlea  is  of  course  filled 
with  perilymph. 

We  have  spoken  as  if  there  were  but  one  membrane 
separating  the  scala  vestibuli  from  the  scala  tympani, 
but  really  there  are  two,  and  these  are  separated  by  a 
little  space  which  is  filled  with  endolymph.  This  space 
is  called  the  scala  media  and  connects  through  the  saccu- 

lus  with  the  vestibular 
sac.  Now  in  the  scala 
media  are  found  a  great 
number  of  stififish  rods 
running  from  the  cen- 
tral pillar  to  the  outer 
wall,  like  stair  rods,  and 
of  diminishing  length  to 

Fig.  114.— Sectian  through  an  ampulla,  the  very  end.  These  are 
showing  a  branch  of  the  auditory  the  rods  of  Corti.  There 
nerve,  n,  and  the  sensitive  hairs,  a  a. 

are  several  thousands 
of  them.  A  branch  of  the  auditory  nerve  runs  up  the 
central  pillar  and  sends  its  fibers  into  the  scala  media, 
and  the  rods  of  Corti  are  supposed  to  be  the  percipient 
terminals  of  these  fibers,  as  are  the  rods  and  cones  of 
the  retina  terminals  of  the  fibers  of  the  optic  nerve. 
Here  again,  therefore,  we  have  a  most  delicate  arrange- 
ment  for  perceiving  the  slightest  vibratory  movement 


SENSE   ORGANS.  l8l 

of  the  lymph.     All  this  membranous  apparatus  is  con- 
nected throughout  (Fig.  115). 

Mode  of  Action  of  the  Whole.— Sound  vibrations 
of  the  air  are  gathered  by  the  conch,  carried  by  the 
meatus    to    the    drumhead,    and    through    the  chain   of 


Fig.  115. — The  whole  membranous  labyrinth:  I's,  vestibular  sac;  s,  the 
sacculus ;  ?>tsc,  memVjranous  semicircular  canals ;  mc,  the  membranous 
cochlea.     (After  Cleland.) 

bones  to  the  sfaj^cs  ;  the  shaking  of  the  stapes  communi- 
cates a  vibratory  motion  to  the  perilymph,  and  this  to 
the  endolymph  of  the  vestibular  sac.  The  shaking  of 
this  causes  the  otoliths  to  agitate  the  nerve  terminals 
exposed  on  the  interior  of  the  sac.  The  vibratory  move- 
ment now  divides  in  several  branches.  Three  of  these 
go  up  through  the  semicircular  canals,  shaking  the  hairs 
of  the  ampullse  in  which  nerve  fibers  terminate.  Still 
another  branch  runs  spirally  up  the  scala  vestibuli  over 
at  the  extreme  end  and  down  spirally  by  the  scala 
tympani.  These  vibrations  are  communicated  to  the 
endolymph  of  the  scala  media  and  impress  the  rods  of 
Corti. 

The  Distinctive  Functions  of  these  Parts. — It 
is  believed  that  there  is  a  distinctive  function  of  each  of 
these  several  parts.  The  vestibular  sac  with  its  otoliths 
seems  especially  adapted  to  perceive  the  slightest  sound 
as  soutid  or  noise,  while  the  cochlea  with  its  rods  of 
graduated  lengths  seems  specially  adapted  to  the  percep- 
tion of  sound  as  tone  or  pitchy  and  therefore  for  the  per- 
ception of  music.     These  rods  might  well   be  supposed 


1 82    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

to  respond  each  to  a  special  rate  of  vibration,  somewhat 
as  a  note  on  a  violin  falling  on  all  the  strings  of  a  piano 
only  one  will  respond,  viz.,  that  one  which  vibrates  in 
unison — co-vibrates — with  the  given  note. 

The  semicircular  canals  with  their  ampullae  and 
terminal  hairs  may  concur  with  the  vestibular  sac  and 
otoliths  in  perceiving  sound  as  noise,  but  they  are  also 
supposed  to  have  another  function. 

We  have  not  yet  drawn  attention  to  the  remarkable 
fact  that  these  three  canals  are  set  in  three  rectangular 
plains,  one  vertical  fore  and  aft,  one  vertical  from  side 
to  side,  and  the  third  horizontal  (Fig.  ii6,  page  183). 
The  least  movement  of  the  head  back  and  forth,  as  in 
nodding,  would  move  the  water  in  the  vertical  fore-and- 
aft  canal  and  would  be  perceived  by  the  hairs  of  its 
ampulla.  Similarly  movements  of  the  head  from  side  to 
side,  as  in  wagging,  would  be  perceived  by  the  ampulla 
of  the  canal  set  in  the  vertical  transverse  plain  ;  while 
rotation  of  the  head  on  the  spinal  column,  as  in  the  sign 
of  negation,  would  be  perceived  by  the  ampulla  of  the 
horizontal  canal.  In  other  words,  these  canals  with 
their  ampulla  are  a  most  delicate  indicator  of  the 
position  and  movement  of  the  head,  and  therefore 
necessary  for  maintaining  the  equi/ibriufn  of  the  body. 
Surely  this  is  a  fundamental  and  most  important  func- 
tion. Many  experiments  seem  to  substantiate  this 
view.* 

The  perception  oi  direction,  which  is  so  mathematically 
exact  in  the  case  of  the  eye,  is  extremely  inexact  in  the 
case  of  the  ear.  It  is  this  inexactness  which  is  utilized 
by  the  ventriloquist  in  producing  his  deceptions. 

*  Some  would  go  further  and  say  this  is  the  sole  function,  and 
others  still  further  and  say  the  only  organ  of  hearing  is  the 
cochlea.     This,  however,  seems  improbable. 


SENSE   ORGANS. 


183 


COMPARATIVE    MORPHOLOGY    AND    PHYSIOLOGY 
OF    THE    EAR. 

In  no  Other  organ  do  we  find  so  regular  a  simplifica- 
tion in  descending  the  scale  of  animals. 

In  mammals  the  structure  and  function  of  the  ear  are 
almost   exactly  what  we  described  in   man.     The  only 
important  differences  are  the  greater  size  and  efficiency 
of  the  external  ear  as  gatherers 
of  sound  waves,  and  the  movable- 
ness  of  the  ear  by  the  use  of  ap- 
propriate muscles  by  which  ani- 
mals   perceive    direction    better 
than   we.      These    muscles    exist 
even  in  man,  but  in  a  rudimentary 
and  therefore  useless  condition. 
The  hearing  of  most  mammals  is 
keener  than  that  of  man,  as  they 
rely  much  on  this  sense  for  their 
safety. 

Birds. — The  first  important 
simplification  is  found  in  birds. 
In  the  exterior  ear  the  conch  is 
entirely  wanting  and  the  meatus 
is  very  shallow,  so  that  the  mem- 
brane of  the  drum  is  very  near  the  surface  of  the  head. 
In  the  mid-ear  the  chain  of  bones  is  reduced  substan- 
tially to  one,  the  columella  (which  represents  the  stapes 
and  probably  the  malleus),  and  the  tympanic  cavity  is 
broadly  connected  with  the  throat  instead  of  by  a  slen- 
der Eustachian  tube.  In  the  interior  ear  we  find  the 
cochlea  much  shorter  and  uncoiled  {¥\g.  116). 

Reptiles. — In  reptiles  the  exterior  ear  is  gone;  the 
membrane  of  the  drum  is  at  the  surface,  covered  with 
skin  and  often  with  muscle.     The.  mid-ear  is  very  similar 


Fig.  116. — Interior  ear  of  a 
bird,  showing  cochlea  {c) 
uncoiled.  The  semicircu- 
lar canals  in  three  rec- 
tangular planes  are  also 
shown.     (From  Parker.) 


l84   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

to  that  of  birds,  but  the  interior  ear  has  lost,  or  nearly 
lost,  the  cochlea. 

In   amphibians  the  whole   of  the  mid-ear  as  well  as 
the  exterior   ear  is  lost ;   only  the  interior  ear  remains, 

A 

B 


■ty 


auv  gc 

Fig.  117. — A,  tibia  of  a  grasshopper  {Meconcfnia)^  with  auditory  organ. 
B,  section  ot  the  same  enlarged  :  t\\  tympanic  membrane  ;  auv^  audi- 
tory vesicle  ;  gc^  ganglionic  cell.     (From  Packard. ) 

and  this  is  reduced  to  vestibule  and  semicircular  canals, 
the  cochlea  being  wanting. 

In  fishes  we  have  still  the  vestibular  sac  and  mem- 
branous semicircular  canals,  but  the  bone  has  not  grown 
completely  about  these  so  as  to  make  bony  cavities  of 
similar  shape;  nor  are  the  cavities  of  the  ear  cut  off 
from  the  brain  cavity. 

Finally  in  invertebrates  the  ear  is  reduced  to  a  vestibu- 
lar sac  and  otoliths.  These,  therefore,  are  the  most  fun- 
damental and  necessary  parts  of  an  organ  of  hearing. 

The  hearing  organs  of  invertebrates,  however,  are 
much  diversified  in  form  and  position.  In  insects  they 
are  found  sometimes  in  the  first  joint  of  the  abdomen,  as 
in  some  grasshoppers ;  sometimes  in  the  lower  joint 
(tibia)  of  the  leg,  as  in  other  grasshoppers;  and  probably 
sometimes  in  the  antennae.  Insects  certainly  hear,  for 
they  produce  sounds  which  are  intended  to  be  heard. 
In  all  the  cases  above   mentioned  there  is  a  hollow  re- 


SENSE   ORGANS.  185 

verberatory  cavity,  with  a  tense  membrane,  ty  (which 
may  be  compared  to  a  membrana  tympani,  or,  better, 
with  the  membrane  of  the  foramen  ovale),  a  vestibular 
sac,  ain\  containing  otoliths  (Fig.  117).  In  crustaceans  \s 
found  a  similar  organ,  sometimes  on  the  anterior  lower 
surface  of  the  cephalothorax,  as  in  crabs,  and  sometimes 
on  the  basal  joint  of  the  antennae,  as  in  lobsters. 

Mollusca. — In  cephalopod  mollusks  the  hearing  organ 
is  in   the  head   just  below  the   brain,  as  a  cavity  in  the 
cartilage  filled  with  endolymph  and  containing  otoliths 
(Fig.  118).     \\\  gastropods 
it    is   a   capsule  of  con- 
densed connective  tissue 
lined     with     epithelium, 
filled  with  liquid  and  con- 
taining otoliths,  situated 
just  below  the  oesophag- 
eal ganglion.*    In  aceph- 
ala  a  similar  capsule  has 
been  found  at  the  base  of 

Fig.  118. — Section  throuf'h  the  head  of 
the    gills    which    is     sup-  a  squid,  showing  the  auditon- organ: 

nncpH    to    hnvP    a    Qimilar  w,  vestibular  sac  and  otoliths ;    eg, 

posea    to    nave    a    similar  cephalic  ganglion  ;  e,  the  eye. 

function. 

Below  this  a  hearing  organ  has  not  been  found. 
Whether  there  be  a  nerve  specialized  for  hearing  is  not 
known,  as  we  can  judge  only  by  the  existence  of  some 
apparatus  like  a  capsule  and  otoliths. 

Thus  far  the  most  essential  part  of  the  ear  is  the 
vestibular  sac  with  its  otoliths.  But  in  spiders  and 
certain  insects  there  is  found  another  type  of  hear- 
ing organs  which  may  be  compared  not  to  the  vestibular 
sac,  but  to  the  hairs  of  the  ampullae.  In  spiders,  on  the 
feelers  are  found  cup-shaped  hollows,  from  the  bottom 

*  Nat.,  iv,  51S  ;  Arch,  des  Sci.,  xliv,  261.  1872. 


1 86   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

of  which  arise  a  fine  tapering  hair,  into  which  runs  a 
nerve  fiber.  The  vibrations  of  the  air  reverberated  in  the 
hollow  determines  corresponding  vibrations  of  the  hair 
and  affect  the  nerve. 

This  style  of  hearing  organ  reaches  its  highest  per- 
fection in  the  mosquito,  especially  in  the  male  mosquito.* 


Fig.  119. — Head  of  a  mosquito.  The  two  lower  and  longer  antenna  are 
feelers,  the  two  upper  ones  with  radiating  hairs  are  auditory.  ( After 
Mayer. ) 

Fig.  119  represents  the  head  of  the  male  mosquito  with 
its  compound  eyes  and  auditory  organs.  There  are  two 
kinds  of  antennae  :  the  one  (the  lower  in  the  figure)  are 
feelers  or  organs  of  touch,  the  other  (the  upper)  are 
organs  of  hearing.  As  in  the  spider,  these  come  from 
the  bottom  of  hollow  cups,  which  probably  act  as  resona- 
tors, increasing  the  air  vibrations;  and  the  long  hair- 
like, many-branching  antennae  respond  by  co-vibrations. 
A  nerve  runs  up  each  antenna  and  sends  a  fiber  to  every 
branch.  Surely  this  is  an  admirable  arrangement  for 
responding  to  aerial  vibrations. 

But,  according  to  the  investigations  of  Mayer  and 


Mayer,  Am.  Jour.,  viii,  81,  1874  ;  Arch,  des  Sci.,  li,  263,  1874. 


SENSE   ORGANS.  1 87 

Johnston,  it  is  also  admirably  adapted  to  appreciate 
both  musical  tone  and  direction.  If  a  mosquito  be  fixed 
under  the  microscope  and  a  sound  be  made  on  a  violin, 
among  the  many  hairs  of  all  lengths  only  a  few  are 
observed  to  vibrate  in  response,  viz.,  those  which  by 
length  are  adapted  to  co-vibrate.  Again,  it  was  observed 
that  in  making  the  sound  in  different  parts  of  the  room, 
of  the  hairs  pointing  in  all  directions  only  those  vibrated 
strongly  which  were  at  right  angles  to  the  direction  of 
the  sound.  They  probably  perceive  direction  much  bet- 
ter than  we  do.* 

Leaving  out,  however,  these  last  contrivances  as  out 
of  the  direct  line  of  evolution,  and  regarding  the  ves- 
tibular sac  and  otoliths  as  the  simplest  form  of  hearing 
organ,  the  simplification  as  we  go  down  the  scale  is 
very  regular  and  may  in  a  general  way  be  expressed  by 
the  following  diagram,  which,  with  the  legend,  will  be 
readily  understood  and  requires  no  further  explanation  : 


Cla.-<scs 

Outer 

Middle 

Inner  Ear              \ 

Mammals 

Birds 

Reptiles 

Amphibians 

Fishes 

Invertebrates 

Conch    1  Meatus 

Tympm.  1   Osaicles 

Bony 
V     sc     c 

Membranous 

-  -)f X--- 

1. 

-Jf- 

IX::: 

--4-— )<---*  -- 

ZTt: 

Tt 

^ 

Fig.  I20. — Diagram  showing  the  g:radual  simplification  of  the  hearing  organ 
as  we  go  down  the  scale.  The  stars  represent  the  presence  of  the  parts 
named  above.  Middle  ear,  i,  2,  .-5,  ossicles;  inner  ear,  F,  .ST,  C,  bony 
vestibule,  semicircular  canals,  and  cochlea ;  f^',  S'C,  C,  membranous 
same  parts.  The  dotted  continuations  of  the  cochlea  line  mean  that 
rudiments  of  cochlea  are  found  in  some  reptiles. 


*  Observe  here  that  the  greatest  effect  is  produced  when  the 
sound-waves  strike  the  sensitive  hairs  broadside,  and  becomes 
nothing  when  it  strikes  end  on.     In  the  case  of  sight  the  very  re- 


1 88    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Therefore  in  the  evolution  of  the  organic  kingdom 
the  most  necessary  and  first  evolved  part  of  a  hearing 
organ  was  the  vestibular  sac  and  otoliths.  These  are 
found  in  the  higher  invertebrates.  Then  in  fishes  there 
were  added  the  semicircular  canals.  Next  the  bone  grew 
about  these  parts,  so  as  to  shut  them  off  from  the  brain 
cavity,  and  at  the  same  time  inclosed  them  in  such  wise 
as  to  form  cavities  of  shape  similar  to  the  organs  them- 
selves. This  is  found  in  the  amphibians.  Then  the  mid- 
ear  was  added,  with  its  tympanic  membrane  and  at  least 
one  ossicle.  This  is  the  case  in  reptiles.  Then  in  birds 
a  cochlea  was  added  and  a  shallow  meatus.  Finally,  in 
mammals  the  cochlea  was  greatly  elongated,  and  there- 
fore coiled,  more  links  were  added  to  the  chain  of  ossi- 
cles, the  whole  apparatus  was  sunk  deeper  into  the  head 
and  with  a  longer  meatus,  and  the  conch  was  added. 

SECTION    VIII. 

Lower   Senses. 

The  three  lower  senses  will  be  more  rapidly  dis- 
patched, because  the  impression  in  these  is  directly  on 
the  specialized  nerve  without  the  intermediation  of  an 
instrument.  We  have  therefore  only  the  specialized 
nerve  and  terminals  to  deal  with,  and  the  affections  of 
these  are  so  inscrutable  that  we  can  have  little  to  say. 

SENSE    OF    SMELL    AND    ITS    ORGAN,    THE    NOSTRIL. 

The  nostril  in  man  is  a  quadrangular  cavity,  passing 
directly  backward  from  the  face  to  the  throat  (Fig.  121). 
It  is  covered  in    front  by  the  overhanging  nose,  but  in 

verse  is  true — i.  e.,  the  effect  is  greatest  when  the  rods  are  struck 
e7id  on.  The  reason  of  the  difference  is  that  sound  waves  are 
waves  of  longitudinal  vibration,  while  light  waves  are  waves  of 
transverse  vibration. 


SENSE   ORGANS. 


189 


Fig.  121. — Vertical  section  through  the  cav- 
ity of  the  nostril :  rw,  roof  of  mouth  ; 
opl,  orbital  plate ;  cpl^  cribriform  plate. 


the  skeleton  it  is  largely,  though  not  entirely,  exposed. 
It  is  bounded  on  each  side  by  parts  of  the  maxillary 

bone,  and  the  orbital 
plates  of  the  ethmoid, 
opl\  above,  it  is  sepa- 
rated from  the  brain 
cavity  by  a  thin  plate 
perforated  with  many 
holes — the  cribriform 
or  colander  plate  of 
the  ethmoid,  cpl — and 
below,  from  the  mouth 
cavity,  by  the  palatal 
plates,  which  form  the 
roof  of  the  mouth  and 
floor  of  the  nasal  cav- 
ity, rm.  It  is  divided 
into  two  symmetric  halves  (right  and  left  nostril)  by 
a  bony  septum,  the 
vomer,  which  is  con- 
tinued into  the  car- 
tilaginous septum  of 
the  nose.  Each  nos- 
tril is  again  divided 
by  the  turbinated  or 
scroll  bones  into  an 
upper  and  lower  part, 
which  differ  in  struc- 
ture and  function. 
The  lower  part  is 
comparatively  sim- 
ple, the  upper  part 
complex  ;  the  one  is 
lined  with  the  ordinary  epithelium  of  the  alimentary 
passages,  the   other   by   a    peculiar   smooth  epithelium. 


Fig.  122. — \'ertical  fore-and-aft  section  through 
one  nostril,  showing  the  olfactorj'  nerve 
coming  down  from  the  olfactory  lobe  and 
the  nerve  of  feeling  coming  from  behind. 


IQO 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


The  former  is  supplied  by  a  branch  of  the  fifth  pair  of 
nerves  (Fig.  122,  //),  which  are  the  nerves  of  common 
sensation  for  the  face;  the  latter  is  supplied  by  the  first 
pair,  or  olfactory  nerve.  The  former  lies  along  the  floor 
of  the  cavity  and  passes  directly  to  the  throat;  the  lat- 
ter lies  above  the  general  passage  to  the  throat  and 
lungs.  The  function  of  the  former  is  breathing,  of  the 
latter  is  smelling.  Each  of  these  have  connections  with 
the  other  cavities,  the  former  with  cavities  in  the  cheek 
bones,  the  latter  with  cavities  m  the  frontal  bones  over 
the  brows.  Inflammation  sometimes  extends  from  the 
nostrils  into  these  cavities. 

Smelling. — The  olfactory  lobes  rest  directly  on  the 
cribriform  plate  of  the  ethmoid,  which  here  forms  a  part 
of  the  floor  of  the 
cranium.  It  sends 
out  a  large  number 
of  nerves  through 
the  holes  of  the 
colander  directly 
down  and  into  the 
nostrils,  and  these 
ramify  over  its 
upper  chambers 
and  their  terminal 
bulbs  are  exposed 
on  the  surface  (Fig. 
122).  Particles  of 
odoriferous  sub- 
stances are  carried  by  the  air  current,  and,  coming  in 
contact  with  the  olfactory  terminals,  produce  the  sen- 
sation we  call  smell.  The  peculiar  form  of  the  terminals 
sensitive  to  odors  is  shown  in  Fig.  123, 

Any    sensation    may    be    involuntary    or   voluntary. 
Thus  seeing  may  be  involuntary,  but  looking  is  voluntary. 


Fig.  123. — Nerve  terminals  of  the  olfactory 
nerves  :  olfc,  olfactory  cells ;  olfnf,  olfac- 
tory nerve  fibers  ;  epc,  epithelial  cells. 


SENSE   ORGANS. 


191 


Hearing  may  be  involuntary,  but  listening  is  voluntary. 
So  smelling  may  be  involuntary,  but  sniffing  is  a  volun- 
tary act  of  smelling.  In  sniffing  we  draw  in  the  air 
suddenly  and  then  stop  it.  The  air  is  thus  forced 
more  thoroughly  into  the  upper  chambers. 

Odoriferous  particles  being  air-borne,  it  is  evident 
that  a  substance  can  not  be  smelled  unless  it  is  volatile. 
The  amount  of  matter  in  the  air  which  may  be  detected 
by  this  sense  is  so  infinitesimally  small  that  it  can  not 
be  estimated.  No  chemical  test  can  compare  with  it  in 
delicacy. 

COMPARATIVE    PHYSIOLOGY    OF    SMELL. 

We  judge  of  the  delicacy  of  this  sense  in  other  ani- 
mals in  three  ways — viz.,  by  the  size  of  the  olfactory 
lobes,  by  the  complexity  of  the  olfactory  surfaces,  and 
by  the  habits  of  the  animal.  Judging  by  either  or  all  of 
these,  there  can  be  no  doubt  of  the  enormous  superiority 
of  mammals  over  man.  This  is  especially  true  of  car- 
nivores, which  hunt  by  smell,  and  of  herbivores,  which 
detect  danger  by  the  same  sense.  Referring  to  Fig.  41 
(page  76),  we  see  at  once  the  great  size  of  the  olfactory 
lobes  in  mammals.  Making  a  transverse  section  through 
the  nostrils  of  a  dog,  a  horse,  or  a  cow,  we  observe  at 
once  the  great  complexity  of  its  chambers.  The  same 
superiority  is  brought  out  still  more  strongly  by  obser- 
vation of  habits.  Think  of  the  keenness  of  the  smell  of 
a  dog,  who  follows  his  master's  tracks  an  hour  after  he 
has  passed  ;  or  of  a  hound  tracking  a  deer ;  or,  again,  of 
a  deer  sniffing  the  air  and  detecting  the  hunter  a  mile 
away.  It  is  probable  that  among  mammals  and  lower 
vertebrates  generally  smell,  not  sight,  is  the  most  impor- 
tant and  most  cbjective  sense.  In  passing  through  a 
strange  country  u<e  take  ocular  notes,  and  may  return 
the  same  way  by  the  use  of  these.  A  dog  under  simi- 
14 


192 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


lar    circumstances    takes   olfactory   notes    for    the   same 
purpose. 

Judging  in  the  same  way,  birds  are  doubtless  supe- 
rior to  man,  though  greatly  inferior  to  mammals.  It  is 
commonly  supposed  that  vultures  are  especially  distin- 
guished for  keenness  of  scent;  but  experiments  of  Au- 
dubon and  Bachman  show  that  they  are  inferior  in  this 
respect  to  dogs.  A  stuffed  deer  attracted  vultures 
from  the  clouds,  but  dogs  paid  no  attention  to  it.  But 
a  real  carcass  concealed  from  view  was  quickly  discov- 
ered by  dogs,  while  circling  vultures  did  not  detect  it. 

In  all  the  lower  land  vertebrates,  such  as  reptiles  and 
amphibians,  the  olfactory  lobes  form  an  important  lobe 
of  the  brain,  and  their  smell  is 
probably  correspondingly  devel- 
oped, but  no  observations  have 
been  made  to  test  it.  We  there- 
fore pass  on. 

Thus  far  vertebrates  are  air- 
breathing,  the  odoriferous  parti- 
cles are  air-borne,  and  the  smell- 
ing organs  are  therefore  con- 
nected with  the  breathing  passages. 
But  fishes  breathe  water,  not  air, 
by  gills,  not  nostrils.  Smells  with 
them  are  therefore  water-borne, 
not  air-borne.  The  organs  in 
fishes  are  two  deep  pits  near  the 
end  of  the  snout,  in  the  usual 
position  of  nostrils,  but  do  not 
yet  open  into  the  throat.  The  interior  of  these  pits  are 
plicated  in  a  complex  manner  (Fig.  250,  page  369),  so 
as  to  increase  the  surface  of  contact  with  odorous  par- 
ticles, and  large  nerves  from  the  olfactory  lobes  are  dis- 
tributed in  them.     In  sharks,  those  bloodhounds  of  the 


Fig.  124. — Brain  of  a  shark  : 
w,  medulla  ;  cb,  cerebel- 
lum ;  o,  optic  lobes  ;  cr, 
cerebrum  ;  <?/,  olfactory 
lobes  ;  wc,  capsule  of  the 
olfactory  nerve.  ( From 
Gegenbaur. ) 


SENSE   ORGANS. 


193 


sea,  the  olfactory  pits  are  particularly  complex,  and  the 
olfactory  lobes  are  of  enormous  size,  often  as  large  or 
even  larger  than  the  cerebrum  (Fig.  124). 

Invertebrates. — That  many  invertebrates,  especially 
insects,  have  a  keen  sense  of  smell  is  clearly  evidenced 
by  the  fact  of  their  being  attracted  by  odors,  as  blow- 
flies by  putrid  flesh  or  butterflies  by  the  fragrance  of 
flowers.  But  where  the  organs  of  this  sense  are  situated 
is  not  certainly  known.  The  reason  why  it  is  so  difficult 
to  determine  is  that  there  is  no  instrument  connected 
with  the  sense  by  which  we  can  know  it.  It  is  probable, 
however,  that  in  the  case  of  insects  it  is  situated  in  the 
antennae.  Insects  are  air-breathing.  The  breathing 
tubes,  as  we  shall  see  hereafter,  penetrate  every  part  of 
the  body,  but  especially  pass  into  and  to  the  end  of  all 
branches  in  the  antennae.  In  spiders,  too,  the  organ  of 
smell,  as  well  as  of  hearing,  is  supposed  to  have  been 
found  in  the  feelers.* 

In  still  lower  invertebrates  the  organs  of  smell  have 
not  been  certainly  detected. 

SENSE    OF    TASTE    AND    ITS    ORGAN,    THE    TONGUE. 

What  we  usually  call  taste  is  a  complex  sensation,  a 
mixture  of  several  sensations.  It  is  impossible  to  dis- 
cuss the  subject  scientifically  without  analysis.  There 
is  usually  a  mixture  of  three  sensations  belonging  to  as 
many  different  kinds  of  nerves — viz.,  common  sensation, 
smell,  and  taste  proper. 

Examples  of  mixture  of  gustation  with  common  sen- 
sation are  numerous.  The  same  batch  of  dough  may 
be  so  mixed  and  baked  that  it  shall  be  heavy  and  stick 
to  the  teeth  in  chewing,  or  may  be  light  and  spongy. 
The  one  we  call  disagreeable,  the  other  agreeable,  to  the 

*  Dahl,  An.  and  Mag.  Nat.  Hist.,  xiv,  329,  1SS4. 


194 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANLMALS. 


taste.  The  true  taste  is  exactly  the  same  in  both ;  the 
real  difference  is  in  the /<?^/ of  the  alimentary  bolus  as  it 
is  chewed  and  moved  about  in  the  mouth.  The  same 
parcel  of  rice  may  be  cooked  thoroughly  and  yet  in  such 
wise  that  each  grain  shall  stand  in  separate  and  self- 
reliant  individuality,  or  in  such  wise  that  all  individual- 
ity is  lost  in  a  common  socialistic  mush.  We  all  know 
the  entire  difference  in  what  we  call  the  taste.  But  really 
it  is  a  difference  in  the  feel  only.  There  is  very  little 
taste  of  any  kind  in  rice,  but  what  there  is  is  exactly 
the  same  in  the  two  cases. 

It  is  far  more  difficult  to  separate  taste  and  smell, 
and  yet  even  popular  language  has  taken  note  of  the 
difference.  It  is  embodied  in  the  two  words  savor  and 
flavor.  Savors  are  tastes,  flavors  are  smells.  \Ve  have 
heard  of  "salt  that  had  lost  its  savor,"*  but  never  its 
flavor,  for  it  has  none  to  lose.  It  is  necessary  to  re- 
member, then,  that  what  we  call  flavors  are  not  tastes  at 
all,  but  smells.  They  are  affections  of  the  olfactory 
nerves,  not  the  gustatory.  They  are  all,  therefore,  vola- 
tile substances,  essential  oils,  compound  ethers,  etc. 
They  can  all  be  smelled  without  eating. 

Examples. — Coffee  or  tea,  so  far  as  taste  in  the  ordi- 
nary sense  is  concerned,  has  two  principles  :  a  bitter 
astringent  principle,  which  is  a  taste,  and  2.  flavor,  which 
is  a  smell.  This  last  is  due  to  a  volatile  substance 
which  may  be  all  driven  off  by  long  boiling.  In  a 
broiled  steak  the  slight  saltiness  is  nearly  all  that  affects 
the  gustatory  nerve.  Its  pleasant  flavor  is  an  affection 
of  the  olfactory  nerve  entirely.  It  can  be  enjoyed  with- 
out tasting  at  all.     The  same  is  true  of  all  fruits.     In  a 


*  The  salt  used  by  the  poor  of  Palestine  is  said  to  have  been 
very  impure — in  fact,  a  sort  of  salty  earth,  from  which  the  salt  was 
easily  washed  out. 


SENSE   ORGANS. 


195 


strawberry  we  have,  first,  the  pleasant  combination  of 
acid  and  sweet.  This  is  taste  proper.  Then  there  is 
also  the  peculiar  flavor  characteristic  of  that  fruit,  which, 
of  course,  is  a  smell.  In  wine — e.  g.,  champagne — we 
have  first  the  pungency  of  the  CO,.  This  is  common 
sensation.  Then  the  pleasant  combination  of  acid  and 
sweet.  This  is  taste  proper.  And  last,  the  character- 
istic flavor,  aroma,  or  bouquet.     This  is  a  smell. 

Examples  of  Tastes. — As  thus  limited  and  defined, 
the  tastes  are  few  in  number,  while  the  flavors  are  al- 
most infinite.  They  are:  (i)  Siceet.  Pure  white  sugar 
has  no  flavor,  and  therefore  no  smell.  Brown  sugar  has, 
because  it  has  a  smell.  (2)  Sour.  Pure  lime  juice,  or 
citric  or  tartaric  acid,  has  no  flavor,  but  a  very  intense 
taste,  but  vinegar  has  a  flavor,  because  it  has  also  a 
smell ;  it  is  volatile.  (3)  Bitter.  Quinine,  or  morphine, 
or  strychnine  has  a  most  intense  taste  but  no  smell,  and 
therefore  no  flavor.  (4)  Salty.  In  the  other  three  there 
is  scarcely  any  variety  at  all  in  each,  but  in  saltiness 
there  is  much  variety,  although  the  taste  of  chloride  of 
sodium  is  the  type.  There  can  hardly  be  said  to  be  any 
other  pure  tastes  besides  these  four. 

The  proof  of  the  essential  difference  between  tastes 
and  flavors  is  found  in  the  fact  that  a  bad  cold,  if  it 
aft'ects  the  upper  chambers  of  the  nostrils,  is  said  to  de- 
stroy the  taste.  It  does  not  destroy  the  taste  of  sugar, 
or  lemon  juice,  or  quinine,  or  salt.  It  destroys  the  sense 
of  smell,  and  therefore  the  appreciation  of  flavors.  If 
we  hold  the  nose,  flavors  are  almost  destroyed ;  not 
entirely,  however,  because  some  will  come  in  by  the 
back  door — i.  e.,  by  the  throat  into  the  nose. 

If  it  be  asked,  If  this  be  so,  why  are  we  not  satisfied 
with  smelling  ?  the  answer  is  plain.  Not  only  is  the  flavor 
stronger  when  taken  as  food,  because  it  then  is  taken  in 
both  ways,  front  door  and  back  door,  but  mainly  because 


igS   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

there  is  in  that  case  a  combination  of  pleasant  sensations. 
For  example,  what  we  call  the  taste  of  a  ripe  peach  is 
first  the  delicious  softness  and  juiciness.  This  is  a 
feeliug.  Then  the  agreeable  combination  of  acid  and 
sw^eet.      This   is   taste   proper.      And   last,  the  whole  is 

glorified  and  crowned 
by  the  exquisite  fla- 
vor, which,  as  already 
shown,  is  a  smell. 

Organ  of  Taste. 
— As  thus  limited,  the 
organ  of  taste  is  the 
tongue,  especially  the 
back  part  of  the  tongue 
and  the  adjacent  parts 
of  the  throat,  and  still 
more  especially  certain 
large  papillae  in  that 
part  of  the  tongue  (Fig. 
125).  There  are  two 
kinds  of  papillae  on  the 
tongue — one  kind  large 
and  flat,  mushroom- 
like,* the  other  small 
and  conical.  Those  of 
the  one  kind  are  found 
only  on  the  back  part 
of  the  tongue  and  are 


Fig.  125. — Interior  of  mouth,  showings  the 
papillae  of  the  tongue  and  the  distribu- 
tion of  the  nerves  of  taste  :  /,  fungiform 
papilla.     (After  Huxley.) 


taste  papillae;  those  of  the  other  are  found  in  every  part, 
but  mainly  near  the  tip,  and  are  tactile  papillae.  There 
are  three  kinds  of  nerves  distributed  to  this  nimble  little 
organ  :  (i)  A  branch  of  the  fifth  pair  (Fig.  t^t,,  5,  page  51). 
This  gives  common  sensation.     It  is  distributed  to  all 


*  These  are  again  subdivided  into  fungiform  and  circumvallate. 


SENSE   ORGANS. 


197 


parts  of  the  tongue,  but  more  and  more  toward  the  tip. 
(2)  The  glosso-pharyngeal  (Fig.  33,  p).  This  also  is  dis- 
tributed to  all  parts,  but  mainly  to  the  back  part  and 

adjacent  parts  of  the  throat. 
This  is  supposed  to  be  the 
special  nerve  of  taste.     (3) 


Fig.   126. — A  fungfiform  papilla,  showing;  the  taste  bulbs  (/6)  of  a  rabbit : 
A,  magnified  ;  B,  highly  magnified,     i^ After  Tuckerman.) 

The  hypoglossal  (Fig.  ;^^,  12).  This  is  a  motor  nerve,  and 
presides  over  the  movements  of  the  tongue.  Through 
these  three  it  becomes  a  tactile  organ,  a  tasting  organ, 
and  a  talking  organ.  The  manner  in  which  the  nerves 
terminate  in  the  taste  papillae  is  shown  in  Fig.  125.  The 
flat  fungoid  papillae  are  eminently  adapted  by  shape  to 
retain  liquids  in  contact  with  the  taste  bulbs. 


COMPARATIVE    PHYSIOLOGY    OF    TASTE. 

There  are  only  two  ways  in  which  we  can  judge  of 
the  keenness  of  taste  in  lower  animals,  viz.,  by  the  or- 
ganization of  the  tongue — i.  e.,  its  softness,  and  espe- 
cially the  development  of  its  papillag,  and  by  observation 
of  the  habits  of  the  animal.  Judging  in  these  ways, 
there  is  probably  little  difference  in  this  regard  between 
man  and  mammals.  The  main  difference  is  that  man 
much  more  than  mammals  takes  food  for  the  enjoyment 


igS    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

of  its  taste.  The  urgency  of  appetite  in  mammals  for- 
bids the  leisurely  enjoyrnent  of  taste  as  such. 

Birds  have  no  teeth.  They  do  not  masticate,  but 
bolt  their  food.  In  many  cases,  too,  their  food,  as,  for 
example,  seeds  and  the  like,  is  hard  and  tasteless.  It  is 
probable,  therefore,  that  their  sense  of  taste  is  more 
imperfect  than  that  of  mammals. 

Reptiles,  atnphibiaiis,  and  fishes  all  swallow  their  food 
without  mastication.  They  have  teeth  ;  but  they  are 
prehensile,  not  masticatory.  We  know  little  of  their 
sense  of  taste,  but  it  is  probably  feeble. 

Of  taste  among  invertebrates  we  know  nothing,  except 
in  the  case  of  insects.  These  are  doubtless  attracted  to 
food  mainly  by  smell;  but  sweets — sugar,  honey,  nectar 
of  flowers — are  sought  and  enjoyed  by  bees,  ants, 
butterflies,  and  flies. 

THE  SENSE  OF  TOUCH  AND  ITS  ORGANS. 

Here,  again,  and  even  more  than  in  the  case  of  taste, 
scientific  discussion  is  impossible  without  analysis.  The 
word  feeling  includes  very  many  distinct  sensations. 
For  example:  lay  the  hand  on  the  table,  palm  upward. 
(i)  Lay  a  card  across  the  finger  tips;  we  have  a  sense  or 
feeling  of  contact.  (2)  Instead  of  a  card  let  it  be  a  ten- 
pound  weight;  we  now  have  in  addition  a  feeling  of 
pressure.  (3)  Let  the  weight  be  fifty  pounds;  we  have 
now  in  addition  a  feeling  of  pain.  (4)  Let  the  weight 
be  hot  or  cold  ;  we  have  now  a  corresponding  feeling  of 
heat  or  cold.  (5)  Now  lift  the  hand  from  the  table; 
in  addition  to  all  the  preceding,  we  have  a  feeling  of 
weight  or  resistance  to  our  muscular  effort — we  feel  the 
heft.  Thus,  then,  there  are  many  kinds  of  sensations  in- 
cluded in  the  word  feeling.  These  are  so  different  that 
they  are  probably  perceived  by  different  specialized 
nerves.     It  is  almost  certain,  according  to  recent  obser- 


SENSE   ORGANS. 


199 


vations,*  that  different  nerve  fibers  take  cognizance  of 
contact,  pain,  heat,  cold,  and  muscular  resistance  (Fig. 
68,  page  95).  In  physics  cold  is  a  negative  term,  a  mere 
absence  of  heat ;  but  in  physiology,  as  we  all  know,  it 
is  a  v^xy  positive  sensation. 

Now,  it  is  impossible  to  take  up  all  the  sensations  in 
detail.  We  take  only  that  which  is  the  most  universal 
and  fundamental,  viz.,  contact  (pressure  being  only  a 
stronger  contact).  This,  together  with  the  muscular 
sense  of  resistance,  gives  us  externality,  or  the  existence 
of  the  external  world. 

Again,  it  is  necessary  to  distinguish  between  general 
sensibility  and  special  sense  of  touch.  The  former,  to- 
gether with  the  so-called  muscular  sense,  gives  the  exist- 
ence of  the  external  world  ;  the  latter  may  be  regarded 
as  the  same,  specially  organized  to  give  definite  knowl- 
edge of  some  of  its  properties,  such  as  shape,  hardness, 
roughness,  etc.  These  two  kinds  of  sense  of  contact  are 
by  no  means  developed  in  the  same  degree.  The  con- 
junctiva of  the  eye  is  exquisitely  sensitive  to  contact, 
but  it  does  not  appreciate  the  properties  of  the  touching 
body  as  does  the  finger  tips  or  the  tongue  tip.  The  same 
difference,  it  will  be  remembered,  we  found  in  the  retina. 
Mere  sensitiveness  to  light  is  keenest  a  little  way  from 
the  central  spot,  but  this  spot  alone  is  specially  organized 
to  give  us  accurate  knowledge  of  shape,  color,  etc. 

General  Organ  of  Touch. — The  general  organ 
for  perception  of  contact  is  the  skin  and  portions  of  the 
mucous  membranes  near  the  outlets  of  the  passages, 
especially  the  mouth.  This  is  for  all  contact.  Besides, 
there  are  certain  portions  of  the  skin  specially  organized 
as  organs  of  touch,  such  as  the  hands  and  the  parts  about 
the  mouth. 

*  Sci.,  vii,  151,  rSS6,  and  459,  1886. 


200   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

General  Structure  of  the  Skin. — The  skin  con- 
sists of  two  parts — dermis  and  epidermis.  The  dermis 
consists  of  fibers  crossing  one  another  in  all  directions, 
and,  as  it  were,  felted  together.  It  is  very  strong  and 
very  highly  organized,  full  of  blood  vessels  and  nerves. 
The  epidermis  contains  no  blood  vessels  or  nerves,  and 
is  not  organized  at  all.  It  consists  wholly  of  epithelial 
cells;  living  nucleated  cells  in  contact  with  the  dermis, 


Fig.  127. — Section  throucfh  the  epidermis  (ep)  and  the  dermis  {d^ :  «,  nerves 
of  touch  terminating-  in  tactile  corpuscles. 


but  becoming  more  lifeless  and  flatter  as  we  go  from  the 
dermis  to  the  surface,  where  they  continually  pass  off  as 
scales  or  scarf.  The  nerve  fibers  come  to  the  epidermis, 
but  do  not  penetrate  it.  They  terminate  near  some  of 
the  cells  of  the  lowest  layer  (Fig.  127). 

Special  Organ  of  Touch. — Wherever  the  skin  is 
specially  organized  for  touch  it  is  thrown  into  ridges, 
as  on  the  finger  tips,  or  else  rises  into  papillae,  as  on 
the  tongue  (Fig.  128).  In  the  case  of  ridges  there  is  a 
row  of  tactile  corpuscles  or  bulbs  in  each  ridge;  in  the 


SENSE    ORGANS. 


20 1 


case  of  papill?e  each  one  contains  a  tactile  bulb.     A  sen- 
sory fiber  enters  and   terminates  in   each  bulb.     These 

are  therefore  end  organs  or 
terminals.  The  epidermis 
proper  is  not  sensitive;  it 
only  protects  the  sensitive 
dermis  from  too  rude  con- 
tact. 

Minimum  Tactile. — 
7     We   have    already  alluded 
to  this  subject  in  compar- 
ing sight  with  touch  (page 
T^        o     T^    .-,  ■  .  132).    We  repeat  it  here  in 

Fig.  128. — Tactile  corpuscles  :  A,  from      . 

the  finger  of  a  man;   H,  from  the      itS    proper    Connection.       If 
skin  of  a  bird;  n.  nerves.     (After  ■        r   :•     -j  t,  j 

Wiedersheim.)  a  pair  01  dividers  be  opened 

widely  and  the  skin  be 
touched  with  the  points  (blunted  a  little  so  that  they  do 
not  prick)  we  feel  two  distinct  impressions.  If  we  now 
bring  the  points  nearer  and  nearer  together,  repeating 
the  experiment  until  we  feel  but  one  impression,  the 
nearest  distance  apart  that  we  can  still 
feel  two  impressions  is  called  the  mini- 
mum tactile.  It  is  a  measure  of  capacity 
to  give  definite  knowledge  by  touch. 
It  differs  greatly  in  different  parts  of 
the  skin.  On  the  middle  of  the  back 
it  is  about  three  inches,  on  the  arm  or 
back  of  the  hand  about  one  half  to 
three  quarters  of  an  inch,  on  the  finger 
tips  about  one  twelfth  of  an  inch,  on 
the  tip  of  the  tongue  about  one  twenty- 
fourth  of  an  inch,  or  one  millimetre. 

Double  Tactile  Images. — There 
is  here  also  a  kind  of  double  images  comparable  to  the 
double  images  of  sight.     If  the  middle  finger  be  crossed 


Fig.  129. 


202    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

over  the  forefinger,  and  a  small  object,  like  a  bullet  or 
a  pea,  be  rolled  beneath  the  crossed  tips,  Hvo  objects  are 
distinctly  felt  (Fig.  129).  It  is  because  the  impressions 
are  on  unaccustomed  or  non-corresponding  points  of  the 
fingers. 

COMPARATIVE    PHYSIOLOGY     OF    TOUCH. 

In  our  rapid  survey  in  passing  down  the  scale,  it  is 
necessary  to  keep  in  mind  the  distinction  between  gen- 
eral sensibility  and  special  sense.  They  are  indeed 
almost  in  inverse  proportions. 

Mammals. — Man  differs  from  mam?iials  chiefly  in  the 
use  of  the  hand  as  an  organ  of  touch.  Indeed  the  fore 
limbs  are  liberated  from  the  function  of  support  and 
progression  expressly  for  this  purpose.  In  other  mam- 
mals, with  the  exception  of  apes,  the  special  tact  organ 
is  localized  elsewhere,  generally  in  the  prehensile  organ 
wherever  that  may  be,  most  usually  about  the  mouth, 
as  the  lips,  the  snout,  the  tongue.  In  such  cases  we 
may  often  find  a  strong  development  of  papillre,  as  on 
the  nose  of  the  dog  or  on  the  sole  of  the  foot  of  all  un- 
hoofed  animals. 

Jn  birds  the  beak  is  an  exquisitely  sensitive  organ  of 
touch.  Sensory  nerves  are  abundantly  distributed  at  its 
base  and  beneath  the  horny  sheath,  exactly  as  in  the 
case  of  our  finger  nails,  which  are  also  delicate  organs 
of  touch.  The  strong  papillae  on  the  feet  of  birds  show 
that  they  are  good  organs  of  touch.  The  general  sensi- 
bility of  birds  is  probably  inferior  to  that  of  mammals 
on  account  of  the  thick  covering  of  feathers. 

Reptiles  are  all  of  them  more  or  less  covered  with 
dry  horny  or  bony  scales  which  must  diminish  their  gen- 
eral sensibility,  nor  have  they  any  well-marked  organ 
specially  organized  for  touch.  They  are  probably, 
poorly  endowed  both  in  general  and  special  sensibility. 


SENSE   ORGANS. 


203 


In  amphibians,  so  far  as  concerns  general  sensibility, 
we  have  the  extreme  opposite  condition — i.  e.,  a  moist, 
active,  sensitive  skin — but  there  are  no  special  organs 
of  touch. 

Fishes  are  probably  similar  to  amphibians  in  regard 
to  general  sensibility.  They  also  have  a  moist,  sensi- 
tive, mucous  surface,  and  therefore  general  sensibility 
well  developed.  In  addition,  many  of  them  have  special 
organs  of  touch  (feelers)  about  the  mouth. 

In  arthropods  we  have  again  the  other  extreme.  They 
all  have  a  hard  skeletal  coat  of  mail  on  the  outside, 
which  almost  entirely  cuts  off  general  sensibility;  but  to 
compensate  they  are  endowed  with  very  delicate  special 
organs  of  touch,  as,  for  example,  the  long  antennae  of 
insects  and  crustaceans. 

In  ftiollusca  we  pass  again  to  the  other  extreme.  A 
universal  characteristic  of  mollusca  is  that  they  are 
everywhere,  except  when  inclosed  in  shell,  covered  with 
a  soft,  active,  mucous  surface,  which  is  endowed  with 
great  sensibility.  Many  also,  in  addition,  have  good 
tactile  organs,  such  as  the  grasping  arms  of  cephalopods 
and  the  so-called  horns  or  feelers  of  snails  and  other 
gastropods. 

In  echiiwderms  the  body  is  again  usually  incased  in 
immovable  shell,  as  in  echinus;  but  again  we  find  com- 
pensation in  their  long,  delicate  tentacles,  which  are 
feelers  as  well  as  locomotive  organs. 

Again  the  pendulum  swings  back  in  ca/e/iferates 
(medusje  and  polyps),  where  we  find  again  the  soft, 
active  mucous  surface  sensitively  responsive  to  contact. 
Their  long  tentacles  also  doubtless  act  as  touch  organs, 
though  perhaps  imperfectly. 

Finally,  m  protozoa  we  find  only  general  sensibility  of 
the  lowest  grade,  to  what  extent  conscious  we  can  not 
tell.     From   this   lowest   form  of  response    to  external 


204 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Stimulus  have  been  differentiated  by  evolution  not  only 
the  different  forms  of  feeling,  such  as  heat  and  cold, 
pain,  and  touch  proper,  but  also  all  the  higher  and  more 
special  forms  of  sense. 

SECTION    IX. 
The  Voice  and  tfs  Organ,  the  Larynx. 

The  voice  is  not  one  of  the  senses,  nor  is  the  larynx 
a  sense  organ,  but  its  close  relation  with  the  sense  of 
hearing  makes  this  the  proper  place  to  take  it  up. 

There  are  three  kinds  of  voice — viz.,  the  call  or  cry, 
the  song,  and  speech.  The  first  is  the  simple  voice,  the 
second  the  harmonically  modulated  voice,  the  third  the  in- 
telligently articulated  \o\q.^.  The  first  is  common  to  all  or 
nearly  all  air-breathing  vertebrates  ;  the  second  is  pecul- 
iar to  man  and  perhaps  birds;  the  third  is  characteristic 
of  man  alone,  although  an  imitated  speech  may  be 
taught  by  man  to  some  birds.     We  take  first 

I.    SIMPLE    VOICE. 

The  Larynx. — Its  Position  and  Relation. — The  organ 
of  the  voice  is  the  larynx.  There  are  two  pipes  leading 
from  the  throat  into  the  cavity  of  the  trunk — the  gullet 
or  ce  sop  hag  us,  and  the  windpipe  or  trachea.  The  one 
leads  into  the  stomach,  the  other  into  the  lungs  ;  the  one 
is  the  passage  for  food,  the  other  for  air  in  breathing. 
The  trachea  is  in  front,  and  may  be  felt  with  the  hand, 
for  it  is  hard,  being  kept  open  by  a  series  of  bony  or 
cartilaginous  rings,  so  that  the  air  passes  through  with- 
out resistance.  The  gullet  is  a  soft,  extensible  pipe, 
collapsed  when  not  occupied  by  food.  Crowning  the 
trachea  and  opening  into  the  throat,  just  behind  the  root 
of  the  tongue,  is  the  larynx.     Nov/,  it  will  be  seen  (Fig. 


SENSE   ORGANS. 


20S 


130),  that  the  passage  of  the  food  from  the  mouth  to  the 
stomach  and  of  the  air  from  the  nostrils  to  the  lungs 
cross  one  another  in  the  throat.  Therefore  there  must 
be  a  valve  which  shall  close  the  larynx  when  the  food  is 
passing,  otherwise  the  food,  especially  liquids,  would  fall 
into  the  larynx  in  swallow'ing.  This  would  produce  great 
irritation  and  pain.  This  valve  is  the  epiglottis.  When 
we  swallow,  the  larynx  is  drawn  up  by  certain  muscles 
with  great  force  toward  the  roof  of  the  throat.  In  this 
position  the  food  in  passing  presses  down  the  epiglottis 
and  closes  perfectly  the  open- 
ing of  the  larynx.    The  rela-  M^^id 

tion  of  the  epiglottis  to  the 
larynx   is   seen   in    Fig.    131. 


Fig.  I  •50.— Section  of  head,  .•show- 
ing- the  relations  of  the  air  pas- 
sage and  the  food  passage  :  g, 
gullet ;  tr,  trachea  ;  rr,  the  point 
of  crossing. 


cr 


Fig.  \%\. — Side  view  of  larynx  :  hy, 
hyoid  ;  epg.  epiijlottis  ;  ///,  thy- 
roid ;  cr,  cricoid  ;  a,  arj-tenoid. 
The  different  parts  are  seen  in 
transparency. 


At  the  junction  of  the  epiglottis  with  the  larynx  there 
are  a  kind  of  cords  (false  vocal  cords),  which  are  sup- 
posed to  have  some  function  in  modifying  the  voice. 

Experiment. — Put  your  finger  on  the  Adam's  apple 
(larynx),  and  try  to  hold  it  down  while  you  swallow.  You 
will  find  it  impossible.     It  rises  in  spite  of  your  effort. 


2o6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Structure.* — Aside  from  the  epiglottis,  the  larynx 
proper  consists  of  four  cartilages,  the  cricoid,  the  thyroid^ 
and  the  two  arytenoids.    The  cricoid  is  a  cartilaginous  ring. 


Fig.  132. — Cartilages  of  the  lar>'nx  :  A,  cricoid  ;  B,  thyroid  ;  C,  arytenoid, 
with  the  vocal  cord  (fc)  attached  ;  a,  crico-thyroid  articulation. 

which  sits  directly  on  the  top  of  the  trachea.  It  is  narrow 
in  front  and  very  wide  behind  (Fig.  132,  A).  The  thyroid  is 
the  largest  cartilage.  Its  singular  irregular  form,  which 
can  hardly  be  described  in  words,  is  seen  in  Fig.  132,  B. 
It  is  bent  on  itself  in  the  form  of  a  V,  opening  back- 
ward, and  the  point  of  the  V  forms  the  point  of  the 
Adam's  apple  in  front.  The  arytenoids  are  two  trian- 
gular cartilages  in  the  form  seen  in  Fig.  132,  C.  These 
four  pieces  are  put  together  in  such  wise  that  the  lower 
posterior  horns  of  the  thyroid  are  articulated,  i.e.,  mov- 
ably  attached  to  the  sides  of  the  cricoid  low  down  {a),  so 
that  the  wide  gap  between  the  two  arms  of  the  thyroid 
V,  \% partly  filled  up  by  the  cricoid.  Now  the  two  aryte- 
noids sit  directly  on  the  top  of  the  cricoid  (Figs.  131 
and  134)  between  the  legs  of  the  thyroid,  and  thus  fill 
up  the  wide  space  more  fully.  This  is  the  skeleton  or 
framework.  The  rest  of  the  larynx  is  made  up  of 
muscles,  except  the  most  important  part  of  all,  which  we 
now  proceed  to  describe. 

*  There  should  be  a  large  model  for  demonstration  of  these 
parts. 


SENSE    ORGANS. 


207 


The  Glottis  and  its  Vocal  Cords. — The  cavity 
of  the  larynx  is  divided  into  an  upper  and  lower  cham- 
ber by  a  transverse  partition — the  glottis.  In  the  glottis 
there  is  a  fore-and-aft  opening — the  rima  glottidis  or  chink 
of  the  glottis.  This  chink  is 
bounded  on  each  side  by  a  ^-^ .  ^.A)L 


Fig.  133. — View  of  the  larynx  from 
above  :  ar,  the  arytenoids ;  cr^ 
the  cricoid ;  c,  vocal  cords  with 
the  rima  or  opening  between. 


Fig.  134. — View  from  behind  :  cr, 
cricoid ;  th,  thyroid ;  a,  aryte- 
noid ;  ep,  epiglottis  ;  hy,  hyoid 
bones. 


firm  tendinous  cord — the  vocal  cords.  These  cords  are  at- 
tached in  front  to  the  V  point  of  the  thyroid,  and  behind 
to  the  two  arytenoids  (Fig.  133,  also  Fig.  131).  The 
tension  of  these  cords  and  the  size  of  the  chink  or  open- 
ing between  them  varies  very  much  under  different  con- 
ditions. This  is  determined  by  observations  with  the 
laryngoscope,  (i)  In  quiet  breathing  the  chink  is  wide 
open,  the  cords  lax,  and  the  breath  comes  and  goes 
noiselessly  (Fig.  135,  A).  (2)  In  aspiration  (sighing) 
the  opening  remains  much  the  same,  but  the  breath  is 
driven  through  with  a  rushing  sound.  The  position  of 
the  cords  is  somewhat  as  seen  in  Fig.  133.  (3)  In 
making  a  vocal  sound  three  changes  are  observed, 
viz.,  the  vocal  cords  are  brought  nearer  together.,  they  be- 
15 


2o8   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

come  tense,  and  the  edges  are  observed  to  vibrate.  (4) 
If  the  vocal  sound  is  high-pitched,  the  chink  becomes 
very  narrow,  the  cords  very  tense,  and  the  vibration 
caused  by  air  driven  through  very  rapid  (Fig.  135,  B). 
(5)  In  the  highest  soprano  head  notes  the  cords  are  still 


Fig.  135. — Glottis  as  seen  with  the  laryngoscope  :  A,  in  simple  breathing  : 
B  and  C,  in  singing  ;   D,  in  straining. 


tenser,  more  pressed  together,  so  that  the  air  is  driven 
through  only  a  small  opening  in  the  middle,  and  the 
vibration  is,  of  course,  still  more  rapid  (Fig.  135,  C). 
(6)  Fmally,  in  violent  straining  or  strong  muscular 
effort  the  glottis  closes  absolutely  air-tight  (Fig.  135,  D). 
We  first  fill  the  lungs,  then  close  the  glottis,  so  as  to  fix 


SENSE    ORGANS.  209 

the   chest    as   a    fulcrum    for    the    action    of    the   great 
muscles  of  the  trunk  and  limbs. 

MUSCLES    OF    THE    LARYNX. 

The  muscles  by  which  these  changes  are  accom- 
plished are  numerous,  some  tightening,  some  loosening 
the  cords,  some  closing  the  chink.  The  following  are 
the  main  ones  : 

Crico-thyroids Censors. 

Crico-arytenoids,  posterior ) 

Thyro-arytenoids }  Relaxers. 

Inter-arytenoids '  Closers. 

Crico-arytenoids,  lateral ) 

Of  the  tensors,  the  crico-thyroid,  as  seen  in  Fig.  130 
arising  from  the  cricoid,  takes  hold  of  the  thyroid  and 
pulls  it  downward,  and  its  upper  part  forward.  The 
crico-arytenoids,  arising  from  the  cricoid  behind,  take 
hold  of  the  arytenoids  and  pull  them  backivard.  These 
may  be  seen  in  Fig.  134,  being  indicated  by  the  dotted 
spaces.  These  two,  the  one  pulling  the  angle  of  the 
thyroid  forivard  and  the  other  pulling  the  arytenoids 
backward,  stretch  the  vocal  cords.  These,  therefore, 
are  the  stretchers  or  tensors  of  the  cords. 

The  relaxers  must  pull  the  thyroid  and  the  arytenoids 
toward  one  another.  This  is  done  by  the  thyro-aryte- 
noids, which  run  fore  and  aft  within  the  larynx  from 
the  thyroid  to  the  arytenoids  just  outside  of  the  vocal 
cords.     They  are  seen  in  Fig.  133. 

T\it  closers  are  of  two  kinds — one,  the  inter-arytenoid, 
runs  from  arytenoid  to  arytenoid  and  brings  these  to- 
gether ;  the  other,  the  lateral  crico-arytenoids,  rotate 
the  arytenoids  in  such  wise  as  to  bring  together  the  for- 
ward projecting  points  to  which  the  cords  are  attached. 
These  can  not  be  well  shown  except  on  a  model. 


210   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Application. — In  quiet  breathing  all  the  muscles 
are  relaxed  ;  the  opening  is  wide,  and  the  breath  comes 
and  goes  quietly.  In  aspiration  the  condition  of  the 
laryngeal  muscles  is  much  the  same,  but  the  breath  is 
driven  more  strongly  by  the  respiratory  muscles  so  as 
to  make  a  rushing  noise,  but  not  a  vocal  sound.  In  mak- 
ing a  vocal  sound  the  tensors  and  the  closers  are  brought 
into  action,  the  cords  are  made  more  and  more  tense  and 
pressed  closer  and  closer  together,  and  the  breath  is 
driven  through  with  greater  and  greater  velocity,  produ- 
cing more  and  more  rapid  vibration  in  proportion  as  the 
pitch  of  the  voice  is  higher.  The  cavities  of  the  mouth 
and  nose  above  and  of  the  trachea  and  bronchi  below 
act  as  resonators  to  increase  the  volume  and  modify  the 
character  of  the  sound. 

2.    SONG. 

We  have  spoken  thus  far  only  of  the  simple  voice. 
Singing  is  only  the  skillful  modulation  of  the  voice  ac- 
cording to  the  laws  of  harmony.  This  is  done  by  skill- 
ful use  of  the  vocal  muscle  in  producing  a  pure  sound, 
and  a  skillful  changing  of  the  play  of  these  muscles  so 
as  to  modulate  the  pitch,  guided  by  the  ear,  and,  lastly,  a 
skillful  modification  of  the  resonant  cavities  of  the  throat, 
mouth,  and  nose.     We  all  know  the  wonderful  result. 

The  Larynx  as  a  Musical  Instrument. — But  what  kind 
of  instrument  is  the  larynx  ?  To  what  shall  we  compare 
it  ?  Some  have  compared  it  to  a  wind  instrument,  espe- 
cially a  tongued  instrument,  like  an  organ  pipe,  or  a 
clarinet,  some  to  a  bird-call.  But  the  favorite  com- 
parison is  with  the  stringed  instrument,  as  is  shown  by 
the  term  vocal  cords.  But  the  least  reflection  is  sufficient 
to  show  that  the  comparison  is  not  true.  The  vocal 
cords  are  only  about  three  quarters  of  an  inch  long  in 
the  male  and   half  an  inch   in   the  female.     Now,  since 


SENSE   ORGANS.  211 

Strings  must  be  tense  in  order  to  vibrate  elastically  at 
all,  and  since,  further,  other  things  being  equal,  they 
make  higher  pitch  in  proportion  as  they  are  shorter,  it 
is  evident  that  strings  of  any  such  length  as  this,  if 
tense  enough  to  vibrate  at  all,  could  only  produce  an 
inconceivably  high  note.  But  see  the  range  of  the  voice  i 
To  what,  then,  shall  we  compare  it  ? 

It  is  strange  that  no  one  has  thought  to  compare  it  to 
an  ordinary  horn — a  stage  horn,  for  example,  or,  better, 
a  French  horn.  In  this  instrument  the  sound  is  modu- 
lated exactly  as  in  the  larynx — viz.,  by  the  tension  and 
the  pressing  together  of  the  lips  of  the  performer.  The 
edges  of  the  rima  glottidis  ought  to  be  called  the  vocal 
lips,  as  indeed  they  are,  and  not  the  vocal  cords,  which 
they  are  not  in  any  sense.  The  analogy  between  the 
two  instruments  is  perfect.  The  performer  on  the  horn 
presses  his  lips  together  tighter,  and  makes  them  tenser 
and  the  opening  between  them  smaller  in  proportion  as 
he  desires  a  higher  note.  He  then  drives  the  air  between 
the  tense  lips  so  as  to  set  their  edges  in  vibration  ;  this 
vibration,  by  alternate  partial  closing  and  opening  of  the 
aperture,  gives  rise  to  successive  jets  or  pulses  of  the 
out-driven  breath,  and  this  in  its  turn  gives  correspond- 
ing pulses  to  the  air  in  the  sounding  cavity  of  the  horn. 
Precisely  the  same,  as  we  have  seen,  takes  place  in  the 
larynx.  The  only  wonder  is  that  so  small  an  instrument 
as  the  larynx  and  the  mouth  cavity  should  be  capable 
of  such  marvelous  effects. 

3.    SPEECH. 

Of  course  the  subject  of  speech  concerns  other  sci- 
ences besides  physiology.  But  the  mechanism  of  the  pro- 
duction of  the  various  sounds  used  in  speech  belongs  to 
physiology  alone.  We  need  no  apology,  therefore,  for 
taking  it  up  briefly. 


2  12    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

There  are  two  kinds  of  speech — viz.,  vocal  and  whis- 
pered. The  one  is  the  articulation  of  the  voice,  the 
other  of  the  aspiratioti. 

Speech  may  be  defined  as  a  succession  of  vowel 
sounds  interrupted  and  separated  by  consonants.  Vowels 
are  modifications  of  the  voice.  Consonants  are  the  modes 
of  interruption.  There  are  two  kinds  of  modifications  of 
the  voice — viz.,  modification  of  pitch,  high  or  low,  and 
modification  of  the  timbre  or  quality  of  the  voice.  The 
former  is  done  in  the  larynx,  as  already  explained;  the 
latter  is  done  by  changes  in  the  mouth  cavity.  The  vowels 
are  modifications  in  timbre,  not  in  pitch.  The  larynx 
has  nothing  to  do  with  it.  We  have  a  good  illustration 
of  what  we  mean  by  timbre  in  the  sounds  of  different 
musical  instruments.  The  same  musical  note  may  be 
made  on  the  flute,  the  clarionet,  the  violin,  or  the  bugle, 
but  how  different  is  the  quality  of  the  sound  in  each  case  ! 

Voicels. — We  give  a  series  of  seven  vowels  in  such 
order  as  to  show  and  easily  describe  the  changes  in  the 
mouth  cavity,  thus:  e,  a,  ah,  au,  d,  oo,  ii.  (i)  Bring  the 
teeth  near  together,  retract  the  lips  a  little,  bring  the 
tongue  forward  until  it  nearly  touches  the  teeth,  and 
then  make  a  sound  with  the  larynx;  the  sound  is  the 
long  ee,  and  can  not  be  anything  else.  (2)  Other  things 
remaining  the  same,  separate  the  teeth  a  little  more  and 
draw  back  the  tongue  a  little,  and  make  a  sound  of  the 
same  pitch  ;  the  sound  now  appears  as  a  \w  fate,  and  can 
not  be  anything  else.  (3)  Open  the  mouth  much  wider, 
draw  back  the  tongue  still  more,  and  again  make  a 
sound  of  the  same  pitch;  it  comes  out  now  as  a  in  far, 
and  it  can  not  be  anything  else.  (4)  Separate  the  jaws 
as  much  as  possible,  draw  back  the  tongue  as  far  as 
possible,  but  bring  the  lips  a  little  nearer  together  in 
front ;  and  the  same  note  now  becomes  au  in  awe.  (5) 
Now  bring  the  jaws  again  a  little  more  together  and  the 


SENSE   ORGANS. 


2T3 


tongue  not  quite  so  much  retracted,  the  lips  drawn  more 
together,  and  a  little  protruded,  and  the  same  note  be- 
comes (?,  as  in  lo !  (6)  Bring  the  jaws  still  more  together 
and  the  tongue  a  very  little  more  to  the  front,  and  the  lips 
more  drawn  together  and  more  protruded,  and  the  same 
note  now  becomes  oo  as  in  tool.  (7)  Finally,  with  all  parts 
remaining  as  in  the  last,  bring  the  tip  of  the  tongue  for- 
ward as  in  the  first  position,  as  in  making  ee.,  and  the 
same  note  now  becomes  //,  or  the  French  //,  or  the  Ger- 
man il  with  the  Umlaut. 

Consonants. — Articulation  is  the  breaking  of  the  voice 
into  segments.  The  vowels  are  the  segments,  the  con- 
sonants the  modes  of  breaking,  or  interruption.  The 
interruption  may  be  complete,  as  in  b,  p,  d,  t,  k,  and  g 
hard,  or  may  be  incomplete,  as  in  s,f,  /,  r,  etc.  The  in- 
terruption may  be  by  the  lips,  as  in  /,  b,  in,  or  between  the 
tongue  and  teeth,  as  in  /,  d,  n,  or  between  the  tongue  and 
roof  of  the  mouth,  as  in  k  and 
g  hard.  Again,  every  one  of 
these  may  be  non-vocalized  or 
vocalized,  so  that  we  may  make 
two  parallel  series  of  conso- 
nants, the  terms  of  which  cor- 
respond each  to  each,  only  dif- 
fering in  the  fact  that  one  is 
vocal  and  the  other  not.    Since 


N^on-vocal .       Vocal. 

P b 

t d 

k g  hard 

s 2 

/ V 

ch J  ^"^  S  soft 

sh j  French 

/// th  soft 


whispered  speech  is  articulation  of  the  aspiration,  it  is 
easy  to  see  why  it  is  difificult  to  distinguish  the  corre- 
sponding terms  of  this  series  in  whispering. 

There    are  other  modes  of   classifying    consonants, 
but  our  object  is  only  to  bring  out  principles. 

COMPARATIVE    PHYSIOLOGY    OF    THE    VOICE. 

Mammals. — The  structure  of  the  larynx  and  the  mode 
of  making  a  voice  is  precisely  the  same  in  mammals  as 


214 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


in  man.  The  only  difference  is  that  by  constant  use  in 
modulating  the  voice  in  speech  and  in  song  the  larynx 
of  man  is  much  more  flexible. 

Birds. — Next  to  man,  birds  have  the  greatest  power 
of  modulating  the  voice,   for  many  of   them  sing  and 
some  may  be  taught  an  imperfect  speech. 
But  both   the  speech   and   the   song  of 
birds  have  an  entirely  different  signifi- 
cance from  that  of  man.     This,  however, 
belongs  to  psychology,  not  physiology.* 
We   should   expect,    then,    that    the 
larynx  of  birds  would  be  highly  devel- 
oped.    On  the  contrary,  it  is  far  inferior 
to  that  of  mammals  (Fig.  136).     But  the 
larynx  is  not  the  organ  of  song  in  birds. 
The  larynx  is  used  only  in  the  simpler 
and   harsher    sounds,    such   as   cries  of 
pain,  distress,  or  anger,  and  perhaps  the 
simple  chirp.     Their  singing  organ  is  an- 
other organ — the  syrinx. 
Syrinx. — The  bird,  then,  has  two  organs  of  voice,  the 
larynx  and  the  syrinx.     The  larynx  is  in  the  usual  place 
at  the  top  of  the  trachea  and  opening  into  the  throat ; 
the  syrinx  is  at  the  lower  end  oi  the  trachea.     It  is  made 
up  of  the  enlarged  lower  rings  of  the  trachea  and  upper 
rings  of  the  two  bronchi.     Fig.  137,  A,  B,  C,  are  different 
views  of  this  organ.     Observe  (i)  that  the  rings  of  the 
bronchi  in  this  part  are  only  a  little  more  than  half  rings, 
and  the  bronchi  are  completed  on  the  inner  side  looking 
toward  one  another  by  a  tense  membrane,  which  acts  as  a 
resonator.      (2)  On  transverse  section  (Fig.  138)  we  see 
transverse  floors  across  the  openings  of  the  bronchi  into 
the  trachea  and  a  true  rima  glottidis  bounded  by  vocal 


Fig.  136.— Larynx 
of  a  bird. 


From  Animals  to  Man,  Monist,  vi,  p.  356,  i5 


SENSE   ORGANS. 


215 


cords  in  each.  (3)  Rising  up  from  the  fore-and-aft  car- 
tilage formed  by  the  union  of  the  two  bronchi,  observe 
a  tense  membrane  with  scythe-like  edge — semilunar 
membrane  (Fig.  137,  C). 

Mode  of  Action. — The  several  rings  of  this  apparatus 
are  movable  on  one  another.  By  appropriate  muscles 
the  resonating  membrane  may  be  made  more  or  less 
tense — the  vocal  cords  may  be  stretched  and  the  lips  of 
the  rima  pressed  together  so  as  to  vibrate  with  various 
degrees  of  rapidity  and  give  rise  to  various  notes  when 
the  air  is  driven  through  them.  The  cavity  of  the 
syrinx,  the  tense  membrane  of  the  bronchi,  and  the 
whole  cavity  of  the  trachea  above  act  as  resonators,  in- 

B 
A 


Fig.  137. — Syrinx  :  A,  front  view  ;  B,  side  view  ;  C,  section  through  the 
lower  part  of  the  trachea  and  between  the  legs  of  the  bronchi,  showing- 
the  resonating  membrane  on  the  inner  side  of  one  bronchus  :  rm,  reso- 
nating membrane  ;  ««,  semilunar  membrane. 


creasing  the  volume  of  the  sound.  The  semilunar  mem- 
brane is  found  only  in  the  best  singers,  and  is  supposed 
to  produce  the  trilling  so  characteristic  of  some  birds. 


2i6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Reptiles. — As  a  class  these  are  silent,  although  a  few 
do  make  sounds  intended  as  calls. 

In  amphibians,  however,  especially  in  frogs,  we  have 
again  animals  abundantly  vocal.  Their  vocal  organs  are 
indeed  very  imperfect,  but  their 
lungs,  which  are  hollow  sacs 
capable  of  great  distention,  act 
as  powerful  resonant  cavities, 
giving  considerable  volume  to 
their  voice. 

Fishes,  being  gill-breathers  or 
water-breathers,    can    have   no 

voice   in  a   proper  sense.      Probably  some  of  them  do 
make  audible  sounds,  but  not  vocal. 

Arthropods  ;  Insects. — True  voice  is  confined  to  air- 
breathing  vertebrates,  and  is  connected  with  respiratory 

passages;  but  if  we  extend 
the  term  to  any  sounds 
intended  to  be  heard  by 
mates,  then  we  may  include 


Fig.  138. — Transverse  section 
just  above  the  bronchi :  vc, 
vocal  cords. 


Fig.   139. — A,  sonant  organ  of  an  orthopter :  rnt,  resonating  membrane  ; 
r,  rasp.     B,  the  same,  magnified. 


insects  also  among  natural  musicians.     If  birds  are  the 
vocalists,  then  are  insects  the  instrumentalists  of  Nature. 


SENSE   ORGANS. 


217 


We  all  know  the  cheerful  chirp  of  the  ^'■cricket  on  the 
hearth,"  the  insistent,  contradictory,  answering  cry  of 
the  katydid,  the  deafening  clatter  of  the  cicada.  The 
organs  by  which  these  noises  are  made  are  very  various 
and  interesting.  Most  of  these  sonant  insects  belong  to 
the  grasshopper  order  (orthopter).  In  these  the  anterior 
pair  of  wings  are  somewhat  hard,  with  strong  stiff  ribs 
and  tense  membrane  between.  Sometimes  the  hind  leg 
is  rubbed  against  ridges  on  the  stiff  edges  of  the  front 
wings.  These  wirelike  ribs  are  especially  stiff  in  the 
overlapping  parts  on  the  back.  Sometimes  these  parts 
in  the  two  wings  are  rubbed  together  with  a  rapid  vibra- 
tory motion,  the  stiff  membrane  between  them  acting  as 
resonators.  Often  a  kind  of  rasp 
is  added  to  produce  more  effect 

(Fig.  139)- 

But  the  most  elaborate  con- 
trivance is  found  in  the  cicada,  a 
homopter.     Fig.  140,  A,  is  the  un- 

vcp 


Fig.  140. — A,  view  of  the  under  side  of  a  cicada  (natural  size)  ;  the  legs  are 
removed  on  one  side  :  vcp,  ventral  cover  plate.  B,  section  a  little  en- 
larged :  vm,  vertical  membrane  ;  w,  muscle  ;  dr,  drumhead  ;  t'r,  ventral 
resonator.     (After  Lloyd-Morgan.) 


der  side  of  the  insect,  natural  size.  Lifting  up  the  ven- 
tral cover  plates,  vcp,  a  tense  membrane  is  disclosed  (w). 
This  is  a  resonant  membrane.  Beyond  this  there  is  an 
enormous  cavity,  almost  filling  the  whole  body,  and  di- 
vided into  two  by  a  thin  vertical  septum  (zv//,  Fig.  140,  B). 


2i8   PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

This  cavity  is  closed  below  by  the  membrane  already 
spoken  of,  vr;  but  above  on  each  side  it  is  closed  by  a 
very  tense  membrane  like  a  drumhead  (Fig.  140,  B,  dr). 
This  drumhead  is  stiffened  by  radiating  ribs  which  are  a 
little  convex  outward.  A  slender  tendon  coming  from 
a  muscle,  m,  in  the  cavity  is  attached  to  the  center  of 
radiation  of  the  ribs.  By  the  action  of  the  muscle  the 
stiff  convex  drumhead  is  drawn  in  with  a  clack,  and 
again  springs  out  with  a  clack  when  the  muscle  is  re- 
laxed. The  muscle  contracts  and  relaxes  with  a  rapid 
vibratory  motion,  and  this  gives  rise  to  the  characteristic 
clattering  noise  of  these  creatures. 

It  is  hardly  necessary  to  say  that  the  humming  and 
buzzing  sounds  so  common  in  insects  while  flying  are 
due  wholly  to  the  rapid  vibration  of  the  wings. 

Animals  in  other  and  lower  departments  are  not 
known  to  make  sounds  intended  to  be  heard. 


CHAPTER    III. 

MUSCULAR    AND    SKELETAL    SYSTEMS. 

We  have  seen  (page  26)  that  four  systems  are  con- 
cerned with  the  distinctive  functions  of  animal  life,  viz., 
the  nervous  system,  the  sense  organs,  the  muscular  sys- 
tem, and  the  skeletal  system.  We  have  finished  the  first 
two.     We  now  take  up  the  second  two. 

The  object  of  both  these  is  to  produce  motion.  But 
motion  is  coextensive  with  life,  and  therefore  not  pecul- 
iar to  animals.  What  is  really  characteristic  of  animals, 
except  the  very  lowest,  is  the  use  of  a  peculiar  apparatus 
of  nerve  and  muscle  to  give  greater  efficiency  to  the 
motion.  In  the  lowest  animals  we  have  only  general 
sensibility  and  general  contractility.  As  we  rise  in  the  scale 
nerve  and  muscle  are  introduced,  but  not  yet  skeleton. 
The  muscle  acts  directly  on  the  body  to  give  motion  and 
locomotion.  Only  in  animals  somewhat  advanced  in  the 
scale  the  skeleton  is  introduced  to  give  greater  velocity 
and  precision  to  the  motion. 

SECTION    I. 
Muscular  System. 

We  have  already  explained  the  /m/c^  called  muscular. 
Its  one  property  is  that  it  contracts  under  stimulus  of 
any  kind.  Now,  a  muscle,  as  an  organ,  is  composed  of 
an  aggregation  of  several  tissues,  of  which  the  muscular 
is   most  abundant  and  characteristic.     But  besides  mus- 

219 


220   PHYSIOLOGY  AND    MORPHOLOGY    OF    ANIMALS. 


cular  tissue  it  has  connective  tissue   to  web  it  together, 
nerves  to  stimulate  it,  and  blood  vessels  to  nourish  it. 

Kinds. — There  are  two  kinds  of  muscle — voluntary 
and  involuntary — differing  from  one  another  in  many 
respects,  {a)  The  one  is  found  on  the  exterior^  the  other 
in  the  interior  of  the  body.  (/->)  The  one  is  red,  the  other 
is  white,  [c)  The  fibers  of  the  one  are  transversely  stri- 
ated, of  the  other  are  non-striated,  {d)  The  nerve  supply 
of  the  one  is  largely  from  the  conscio-voluntary  system, 

of  the  other  from  the  reflex 
system,  {e)  In  the  voluntary 
muscle  the  fibers  are  massed 
into  a  distinct  organ,  having 
a  distinct  name,  and  all  co- 
operate through  a  tendon  to 
produce  one  motion ;  while 
the  involuntary  muscle  exists 
in  sheets  of  parallel  fibers, 
surrounding  hollow  organs 
(stomach,  intestines,  bladder, 
etc.),  has  no  tendon,  and  the 
fibers  do  not  contract  co- 
operatively, but  consecutively, 
by  a  contraction  propagated 
from  fiber  to  fiber.  (/)  The 
Fig.  141.— Muscular  fibers  of  the  voluntary  muscles  contract 
jAaerlentr'*""^"  "'°"''^^'  quickly  and  powerfully,  the 
involuntary  slowly  and  fee- 
bly, {g)  Lastly,  the  voluntary  are  attached  to  the  skele- 
ton, while  the  involuntary  are  not  attached  to  the  skele- 
ton, but  surround  hollow  organs. 

There  are  some  muscles,  howe»ver,  which  are  inter- 
mediate. The  most  striking  case  is  the  heart.  The 
muscle  of  the  heart  is  red,  transversely  striated,  and 
contracts  powerfully,  and  yet  it  is  /V/voluntary,  consists 


MUSCULAR   AND    SKELETAL   SYSTEMS.  22  1 

of  parallel  fibers,  and  surrounds  a  hollow  organ  without 
skeletal  attachment. 

A  muscular  fiber  is  apparently  evenly  cylindrical, 
without  any  evidence  of  cellular  origin.  But  in  embry- 
onic development  it  is  seen  to  be  formed  by  a  coales- 
cence of  elongated  nucleated  cells.  This  is  well  seen 
in  the  fibers  of  the  embryonic  heart  of  the  monkey 
(Fig.  141). 

Voluntary  Muscle. — Form. — The  typical  form  of  a 
voluntary  muscle  is  seen  in  the  muscles  of  the  limbs — 
e.g.,  the  biceps — which  is  shown  in  Fig.  146,  page  228. 
It  is  attached  to  the  skeleton  at  both  ends.  The  nearer 
and  more  fixed  point  is  called  the  origin,  the  farther  and 
more  movable  point,  the  insertion.  Between  the  two 
the  largest  and  most  contractile  part  is  called  the  belly. 
The  fibers  all  unite  to  forrn  the  tendon,  by  which  it  is 
attached  to  the  skeleton.  This  is  the  type,  but  there  is 
considerable  variation.  Sometimes  the  fibers  are  con- 
vergent. This  is  mainly  in  muscles  connecting  the  limbs 
with  the  trunk,  as  in  the  deltoid,  the  pectoral,  etc. 
Sometimes  the  fibers  are  nearly  parallel,  as  in  the  mas- 
seters. 

Structure. — x\  voluntary  muscle  is  a  definite  mass  in- 
vested by  a  thin  fibrous  membrane — sheath.  If  cut  into  we 
find  it  made  up  of  bundles  of  fibers — fasciculi  {Y'\g.  142,  A). 
These  are  conspicuous  in  cured  meat,  such  as  corned 
beef.  They  constitute  the  grain  of  the  flesh.  These, 
too,  are  invested  with  a  thin  membrane  of  fibrous  tissue. 
These  bundles  are  in  their  turn  composed  of  fibers  lying 
parallel  to  one  another  in  the  bundle.  Each  fiber  is 
also  invested  with  a  very  thin  sheath  of  fibrous  tissue. 
The  fibers  themselves  are  supposed  by  some  to  be  com- 
posed of  smaller  fibrillar,  but  this  is  doubtful.  Fig.  142,  B, 
represents  a  single  fiber,  broken  and  twisted,  showing 
the  sheath.     We  may  regard  the  whole  muscle  as  pene- 


222   PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


trated  and  webbed  together  in  all  its  parts  with  con- 
nective tissue.  In  a  condensed  form  it  invests  each 
fiber  ;  in  a  loose  form  it  lies  between  and  connects  them. 
In  a  condensed  form  it  again  invests  the  bundles;  in  a 
loose  form  it  lies  between  and  con- 
nects these  also.  Finally,  it  emerges  C 
on   the  surface,   and   in    a   condensed 


Fig.   142. — A,   a  fascicle  of  muscular  fibers  of  voluntary  muscle ;    B,  one 
fiber  broken  to  show  its  investing  sheath  ;  C,  cells  of  involuntary  muscle. 

form  invests  the  whole  muscle  in  a  strong  sheath,  and 
in  a  loose  form  lies  between  the  different  muscles,  con- 
necting yet  separating  them.  Add  to  this  the  blood 
vessels  to  nourish  and  nerves  abundantly  distributed  to 
stimulate  to  contraction,  and  we  have  a  good  general 
idea  of  the  organ  we  call  a  muscle. 

The  tendon  consists  of  all  the  sheaths  of  the  muscle, 
of  the  bundles,  and  of  the  fibers  united  and  continuing, 
and  also  of  the  transformed  fibers  themselves.  It  is 
therefore  essentially  fibrous  in  structure.  It  is  the 
strongest — /.  <?.,  has  the  greatest  tensile  strength  of  any 
organic  substance  known. 

The  only  function  of  a  muscle  is  to  contract.     This, 


MUSCULAR   AND    SKELETAL    SYSTEMS.  223 

indeed,  is  the  only  sign  of  its  life.  Any  stimulus — me- 
chanical, chemical,  or  electrical — may  cause  it  to  con- 
tract, but  the  normal  physiological  stimulus  is  the  nerve 
influence,  whatever  that  may  be.  When  a  muscle  con- 
tracts it  shortens,  thickens,  and  hardens.  The  power 
with  which  it  pulls  in  shortening  is  almost  incredible. 

Involuntary  Muscle. — We  have  already  sufficient- 
ly characterized  this  in  contrasting  it  with  the  voluntary. 
Good  examples  of  these  are  found  in  the  muscular  coats 
of  the  stomach,  intestines,  and  bladder.  Every  one  is 
familiar  with  this  type  in  the  white  muscular  substance 
of  tripe. 

No  comparative  physiology  of  this  system  is  neces- 
sary, as  the  function,  structure,  and  mode  of  action  of 
muscle  is  much  the  same  in  all  animals,  as  far  as  the 
tissue  can  be  traced.  Striation  of  muscular  fiber  is  a 
sign  of  great  activity,  and  is  therefore  found  only  in 
animals  of  considerable  energy.  It  is  conspicuous  in 
vertebrates,  and  among  invertebrates  in  arthropods, 
especially  insects.  As  we  go  down  the  animal  scale 
muscular  fiber  is  found  as  low  as  the  coelenterates — 
medusae  and  polyps.  In  protozoa  it  is  replaced  by 
general  contractility  of  protoplasm. 

SECTION  n. 

Skeletal  Systetn. 

The  skeleton  in  vertebrates  is  usually  knie,  because 
this  is  the  most  rigid  of  the  tissues.  In  the  lower  fishes, 
however,  and  in  the  embryonic  condition  of  all  verte- 
brates cartilage  takes  the  place  of  bone. 

Bone   is    tissue   hardened   by  deposit  of    lime    salts, 

mainly  phosphate.     Several  kinds  of  tissue  take  on  this 

change.    Thus,  as  to  origin,  we  may  have  cartilage  bone, 

tendon  bone,  and  skin  bone.     Most  of  the  true  skeleton 

16 


224 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


consists  of  cartilage  bone,  but  the  kneecap  and  several 
small  bones  in  the  joints  are  tendon  bones,  while  the 
teeth  and  scutes  are  examples  of  skin  bones.  Bones 
are  covered  with  a  strong  membrane  called  the  peri- 
osteum. 

Number  of  Bones  in  Man. — The  number  of  bones 
in  the  human  skeleton  is  variously  estimated  from  a  little 
over  two  hundred  to  about  two  hundred  and  fifty,  the  dif- 
ference being  the  result  of  difference  of  view  as  to  what 
ought  to  be  included.  Some  include  the  teeth,  and  some 
do  not,  because  these  are  gum  structures.  Some  include 
the  sesamoid  or  joint  bones,  some  do  not.  Agam  many 
pieces  are  separate  in  the  embryo,  and  become  consoli- 
dated later.  These  two  hundred  or  more  pieces  are  ar- 
ticulated together  into  a  complex  structure,  which  is 
moved  by  the  muscles. 

Joints. — The  articulations  or  joints  are  of  two  gen- 
eral kinds,  _/?.w^  and  movable.  In  fixed  joints  the  pieces 
may  be  cemented  together  with  cartilage  {symphyses), 
as  the  hip  bones  with  the  sacrum,  or  they  may  be  inter- 
digitated  or  dovetailed  {suture),  as  in  the  bones  of  the 
skull.  The  movable  joints  are  also  of  two  kinds — the 
hinge  Joint,  when  motion  in  one  plane  is  required,  as  in 
the  knee,  elbow,  etc.,  and  the  ball-and-socket  Joint,  where 
universal  motion  is  required,  as  in  the  shoulder  and  hip. 
Only  the  movable  joints  concern  us  here. 

Movable  Joints. — The  bones  in  a  movable  joint 
are  {a)  enlarged  at  the  ends  where  they  come  together, 
so  as  to  make  a  firmer  contact,  {b)  They  are  covered 
with  cartilage  for  perfect  smoothness  and  elasticity,  {c) 
The  joint  is  inclosed  in  a  capsule  of  strong  fibrous  tis- 
sue to  prevent  displacement,  and  at  the  same  time  ex- 
clude the  air.  {d)  The  closed  cavity  thus  formed  is 
lined  with  a  serous  membrane,  which  secretes  a  smooth, 
glairy,  lubricating  fluid — the  synovia  or  joint  juice. 


MUSCULAR   AND   SKELETAL   SYSTEMS. 


225 


All  of  these  characters,  intended  for  easy  motion  and 
to  prevent  dislocation,  are  common  to  all  movable  joints; 
but  to  these  in  the  case  of 
hinge  joints  are  added  (<?) 
two  strong  ligaments,  one 
on  each   side,  which    hold 


Fig.  143. — Diagrammatic  view  of 
the  knee  joint.  The  dotted  lines 
show  the  position  of  the  capsule 
and  of  the  lateral  lij^aments  ;  the 
black  lines  show  the  crossed  liga- 
ments. 


Fig.  144. — Section  through  the  hip 
joint,  showing  the  capsule  (cl) 
and  the  round  ligament  (rl). 


the  parts  together  and  yet  allow  free  motion  in  one 
plane.  These  are  the  lateral  ligametits.  They  are  found 
in  all  hinge  joints.  In  addition  to  these  (/),  in  the 
knee  joint  there  are  two  crossed  ligaments  within  the 
joint  itself,  as  shown  in  Fig.  143. 

In  ball-and-socket  joints  we  have,  of  course,  d:,  b,  c,d ; 
but  in  the  case  of  the  hip  joint,  in  addition  to  these, 
there  is  a  short,  strong,  round  ligament  running  from 
the  bottom  of  the  deep  socket  to  the  top  of  the  ball,  as 
shown  in  the  diagram  (Fig.  144). 

SOME    EXAMPLES    OF    ADAPTATION. 

The  articulated  skeleton  together  forms  a  really 
wonderful  contrivance.  Some  examples  of  adaptation 
may  be  mentioned.  I  select  such  as  lead  to  interesting 
comparisons  with  other  vertebrates. 


226  PHYSIOLOGY   AND   MORPHOLOGY   OF   ANIMALS. 


I.  Spinal  Column. — Observe  (i)  the  double  or  S  curva- 
ture. This  is  peculiar  to  man,  and  the  result  of  the 
erect  attitude.  In  this  position  it  acts  as  a  spring  to 
prevent  shocks  to  the  brain  in  falling,  leaping,  etc.  Ob- 
serve (2)  the  intervertebral  substance.  This  is  an  elas- 
tic cushion,  of  half  an  inch  thickness,  between  all  the 
vertebrae,  and  also  acts  to  prevent  concussion  of  the 
brain.     It  is,  however,  not  peculiar  to  man. 

Comparative  Morphology  of  the  Column. — The   manner 
of  articulation  of  the  vertebrre  is  very  characteristic  of 

the  different  classes  of  ver- 
tebrates. In  mammals  the 
vertebrae  are  short  seg- 
ments of  a  cylinder  with 
flat  faces  and  intervertebral 
substance  between  (Fig. 
145,  a^.  In  reptiles  the  ar- 
ticulation is  by  ball-and- 
socket  joint.  The  faces  of 
each  vertebra  are  one  of 
them  concave  and  the  other 
convex,  and  these  fit  over 
one  another,  b.  In  some 
reptiles  the  hollow  face 
looks  forward  (procoelian)^ 
in  others  backward  (opisthocoelian).  In  fishes  the  seg- 
ments are  double  concave  (amphicoelian),  with  double 
convex  intervertebral  substance  between,^.  Some  early 
reptiles  were  like  fishes  in  this  regard.  In  the  lowest 
fishes  there  is  a  continuous  unsegmented  cord  (noto- 
chord),  d.  This  is  also  the  early  condition  of  all  ver- 
tebrates, whether  in  the  embryonic  development  or  in 
the  evolution  of  vertebrates.  This  early  notochord  is 
afterward  changed  into  a  vertebral  column  by  ossifica- 
tion  in  segments.      These   characteristics  of  the   verte- 


ffO  0  Q  Q  0  0  Q 


Fig.  145. — Diagfram  showing  the 
characteristics  of  the  vertebral 
column  in  mammals  (a),  reptiles 
((5),  fishes  (c),  and  lowest  verte- 
brates (d). 


MUSCULAR   AND    SKELETAL   SYSTEMS.  227 

bral  column  in  different  classes  are  very  important  in 
geology. 

2.  Structure  of  Shoulder  Joint  and  Fore  Limb. — There 
is  a  progressive  change  from  man  to  the  more  specialized 
mammals  in  the  position  of  the  shoulder  joint,  its  mov- 
ableness,  the  presence  and  movableness  of  the  tivo  bones 
of  the  forearm,  and  the  free  use  of  the  hand.  All  these 
are  strictly  correlated,  and  all  reach  their  highest  point 
in  man,  because  all  are  in  him  connected  with  the  erect 
attitude  and  the  consequent  liberation  of  the  fore  limbs 
from  the  function  of  support  and  locomotion  for  higher 
uses.  In  man  the  shoulder  joint  and  the  arms  are  on 
the  side  of  the  body,  being  kept  wide  apart  by  the  clavi- 
cle, and  the  motion  of  the  arm  is  the  extreme  of  free- 
dom. For  this  freedom  firmness  of  the  joint  is  sacrificed. 
The  two  bones  of  the  forearm  roll  the  one  on  the  other, 
carrying  the  hand  with  it,  and  the  hand  has  the  most 
perfect  capacity  for  grasping.  Now,  as  we  go  down  the 
scale  of  mammals,  the  collar  bone  disappears,  the  fore 
limbs  are  brought  together  in  front  for  support,  the  two 
bones  of  the  forearm  become  less  and  less  movable  on 
one  another,  and  the  paw  loses  its  power  of  grasping,  un- 
til, fiinally,  in  the  hoofed  animals  the  extreme  is  reached  ; 
the  limbs  are  brought  closer  together  in  front  and  used 
only  for  support,  and  therefore  restricted  in  motion  ; 
the  two  bones  of  the  forearm  are  consolidated  into  one, 
and  therefore  lose  entirely  all  rotary  motion  ;  the  paw 
is  no  longer  a  paw,  much  less  a  hand,  but  a  hoof,  wholly 
incapable  of  grasping. 

Motion  and  Locomotion. — We  have  explained  the 
function  of  muscle  and  of  skeleton.  We  must  now 
show  how  they  co-operate  to  produce  motion  and  loco- 
motion. 

I.  Limb  Motion. — Remember,  then,  that  a  muscle  has 
two  skeletal  attachments — viz.,  the  origin  and  insertion. 


228    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Between  these  there  is  always  a  joint.  When  the  muscle 
contracts,  its  two  attachments  are  brought  nearer  to- 
gether, and  the  joint  bends.  Remember,  again,  that  a 
muscle  can  only  /////,  not  push,  and  therefore  motion 
in  two  directions  can  only  be  effected  by  two  opposing 
muscles.  Again,  it  must  be  borne  in  mind  that  in  ani- 
mal mechanism  power  is  nearly  always  sacrificed  \.o  vetoc- 
//v,  because  swiftness  is  more  valuable  in  the  struggle  for 
life  than  slow  dead  strength.  Therefore,  of  the 
different  orders  of  levers,  the  second  is  rarely, 
almost  never  used,  and  in  the  first  order  the 
fulcrum  is  so  placed  that  power  is  sacrificed  to 
velocity. 

Examples  of  the  first  order  are  very  numer- 
ous.    The  action  of  the  triceps  on  the  point  of 
the  elbow  in  straightening  the  arm  and  of  the 
gastrocnemius   on   the   heel    bone    in    bringing 
down  the  toes  (Fig.  148) 

j-^      are     excellent    examples. 

Examples  of  the  third  or- 
)  der  are  equally  numerous. 
We  again  take  two :  i .  Ac- 
tion of  biceps,  pulling  on  its  insertion,  and  flexing  the 
elbow  (Fig.  146).  2.  The  action  of  the  deltoid  on  its 
insertion,  in  raising  the  arm  (Fig.  147). 

The  prodigious  force  of  muscular  contraction  may 
be  easily  calculated  from  these  examples.  Take  the 
case  of  the  biceps  in  bending  the  arm  at  the  elbow.  I 
suppose  any  one  with  average  muscular  vigor  can  hold 
fifty  pounds  in  the  hand  with  the  elbow  joint  at  right 
angles,  as  in  Fig.  146.  In  such  case,  taking  the  distance 
of  the  insertion  of  the  tendon,  /',  from  the  fulcrum,/,  as 
one  inch,  and  from  the  fulcrum  to  the  weight,  W,  one  foot, 
the  pull  of  the  muscle  on  the  insertion  at  P  necessary 
to  hold  up  the  weight  would  be  50  X  12  =  600  pounds. 


Fig.  146. — Diagram  showing 
the  power  of  the  biceps  in 
flexing  the  arm. 


Muscular  and  skeletal  systems. 


229 


Or  take  the  case  of  the  deltoid  holding  a  weight  at 
arm's  length  (Fig.  147).  I  suppose  a  man  of  good  mus- 
cular vigor  will  hold  at  arm's  length  a  weight  of  thirty 


'flC~"  ■ "  -^ 


:2^ 


Fig.   147. —  Diagram  showing;  the  power  of  the  deltoid  in  raising        ,^, 
the  arm. 


pounds.  Now,  in  order  to  do  so,  the  deltoid  taking  hold 
at  four  inches  from  the  fulcrum,  /,  and  the  weight  held 
at  two  feet  from  the  same,  if  it  pulled  directly  upward  it 
would  have  to  pull  with  a  force  of  30  X  6  =  180  pounds. 
But  it  pulls  at  a  small  angle,  and  we  must 
multiply  this  again  by  at  least  four — i.  e., 
180  X  4  =  720  pounds. 

Or  take  one  more — viz.,  the  case  of  the 

gastrocnemius  and  soleus  muscles  (the  calf 

the  leg)  lifting  the  heel.     I  suppose  any 

erson   of  ordinary   weight  and   vigor  can 

e   another    person    of    average    weight, 

one  hundred  and  fifty  pounds,  on  his 

back,  making  altogether,  say,  three  hundred 

nds,  and,  standing  on  one  foot,  rise  to 

06.    Let  us  see  what  the  strain  on   the 

tendo-Achillis  is  in  doing  so. 

Taking  the  distance  from  the 

fulcrum  (2,  Fig.  148)  to   the 

insertion  of  the  tendon,  /,  as 

one    inch,    and    the    distance 

from  the  same  to  the  ball  of 

the  toes,  j,  as  si.x  inches,  then  we  have  the  proportion  /,  2 

=  one  inch  :  .2,  j  =  6  inches  : :  300  pounds  :  x,  and  x  =: 

1,800  pounds.    Or  if  some  one  objects  (as  has  been  often 


Fig.  14H. — Diaf;;ram  showing  the 
power  of  the  gastrocnemius  and 
soleus  muscles  in  tiptoeing. 


230 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


objected)  that  j  must  be  taken  as  the  fixed  point  or 
fulcrum,  and  therefore  we  must  treat  the  question  as 
falling  under  the  second  order  of  levers — i.  e.,  the  power 
pulling  upon  the  lever  /,  j>  =  7  inches,  and  the  weight 
pushing  down  on  the  lever  2,  j  =  6  inches,  then  we  must 
remember  that  the  muscle  pulls  the  body  downward, 
adding  to  the  weight  exactly  as  much  as  it  pulls  the 
heel  upward.  Therefore,  under  this  conception,  we  must 
put  the  proportion  thus — 7  :  6  ::  300  -\-  x  :  x.  yx  =  6x  -\- 
1,800 — or  X  =  1,800. 

It  has  been  determined  by  experiment  that  one  square 
inch  of  muscle  will  contract  with  a  power  equal  to 
about  one  hundred  to  one  hundred  and  twenty  pounds. 
Why,  then,  does  not  the  muscle  break  ?  For  dead 
muscle  can  stand  no  such  tensile  strain  as  this.  The 
answer  is  :  It  would  break  if  it  were  passive,  and  the 
force  was  external  to  itself,  but  it  is  the  attraction  be- 
tween the  molecules  of  the  muscle  itself  that  develops 
the  pull ;  attraction  can  not  produce  separation.  Muscles 
do  break  sometimes,  but  always  from  irregular  contrac- 
tion— i.  e.,  one  part  contracts  while  another  part  does 
not,  and  is  therefore  subject  to  tensile  strain,  just  as  the 
tendon  is. 

2.  Locomotion. — In  limb  motion  the  origin  or  body 
end  of  the  muscle  is  fixed,  and  the  insertion  or  limb  end 
moves;  but  these  are  interchangeable.  If  we  fix  the 
limb,  then  the  body  moves.  Thus,  for  example,  if  we 
hold  up  the  hands  above  the  head  and  bring  into  action 
the  great  muscles  about  the  armpit,  and  also  the  biceps, 
the  elbow  is  brought  down  to  the  side  and  the  fist  to 
the  chin  ;  but  if  we  fix  the  hands  by  taking  hold  of  a 
bar  and  bring  into  action  the  satne  muscles,  the  body 
rises  until  the  chin  goes  over  the  bar.  Now  locomotion 
is  nothing  more  than  limb  motion  reacting  against  the 
ground  in  walking,  running,  leaping,  against  the  water 


MUSCULAR    AND   SKELETAL   SYSTEMS.  23 1 

in  swimming,  and  against  the  air  in  flying.  For  instance, 
let  a  steam  engine  be  lifted  from  the  rails  and  steam  be 
put  on ;  we  have  only  wheel  motion.  But  while  thus 
working  set  the  engine  on  the  track,  and  wheel  motion 
is  changed  into  locomotion.  Or  lift  a  cyclist  above  the 
ground  and  let  him  work  his  pedals  ;  we  have  now 
'  limb  motion  and  wheel  motion.  But  set  him  on  the 
ground,  and  away  he  shoots,  scorching  the  ground  as  he 
passes;  wheel  motion  is  converted  into  locomotion.  Or, 
again,  take  the  sprinter,  hang  him  up  in  the  air,  and  let 
him  set  his  running  muscles  into  action.  We  have,  of 
course,  only  extravagant  limb  motion.  But  while  this 
is  going  on,  if  we  set  him  down  on  the  earth,  instantly 
limb  motion  is  converted  into  rapid  locomotion.  There 
is  therefore  no  new  principle  involved,  and  no  further 
discussion  required. 

But  there  is  still  one  point  which  must  be  mentioned 
— viz.,  the  exquisite  co-ordination  of  action  of  many  mus- 
cles required  in  nearly  all  our  motions.  For  example, 
in  the  simple  act  of  standing  there  are  probably  at  least 
one  hundred  muscles  in  perfect  co-ordinate  action  to 
maintain  the  equilibrium.  It  is  so  easy  and  so  instinct- 
ive that  we  are  unconscious  of  the  constant  play  of 
many  muscles.  If  so  in  standing,  how  much  more  in 
walking,  running,  leaping,  swimming,  flying !  So  won- 
derful, indeed,  is  this  co-ordination  that  it  could  not  be 
learned  in  a  lifetime  if  it  were  not  largely  inherited.  A 
calf  newly  born  will  stand  on  its  feet  and  walk.  It  has 
not  learned  to  do  so,  but  has  inherited  the  capacity.  A 
chick  newly  hatched  will  walk  and  use  its  eyes  correctly 
and  peck  its  food.  A  wild  bird's  egg  may  be  taken 
from  the  nest,  hatched  in  an  incubator,  and  reared  in  a 
cage  until  the  young  bird  is  well  feathered,  until  nerves 
and  muscles  are  sufficiently  developed.  If  then  it  be 
carried  out  and  thrown  m  the  air  it  will  at  once  flv  awav 


232    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

with  ease.  Even  a  child,  when  the  proper  time  comes — 
i.  e.,  when  the  nervous  and  muscular  systems  are  suffi- 
ciently developed — will  learn  to  walk  in  a  week.  Even 
in  the  child  a  large  portion  of  this  capacity  of  co-ordina- 
tion is  inherited,  though  far  less  than  in  animals.  The 
whole  sum  of  capacity  in  all  animals  is  partly  inherited 
and  partly  individually  acquired.  In  animals,  and  al- 
most in  proportion  as  they  are  lower  in  the  scale,  the 
inherited  part  is  large  in  proportion  to  the  acquired 
part.     In  man  the  reverse  is  true. 


SECTION    III. 

Coviparative  Morphology  and  Physiology  of  Muscle  ajid 
Skeleton. 

VERTEBRATES. 

So  far  as  vertebrates  are  concerned,  the  function  of 
muscle  and  skeleton  is  almost  identical  with  that  already 
explained  in  man,  although  there  is  great  variation  in 
the  structure  by  means  of  which  function  is  carried  out. 
But  this  will  be  brought  out  in  a  separate  chapter  on 
the  laws  of  animal  structure  in  relation  to  the  origin  of 
organic  forms  by  evolution.  We  pass,  therefore,  directly 
on  to  the  invertebrates. 

INVERTEBRATES. 

The  function  of  motion  in  invertebrates  is  so  infi- 
nitely various  that  all  that  is  possible  in  this  work  is  to 
give  some  characteristic  examples  of  widely  different 
modes.  The  most  interesting  of  these  is  that  of  ar- 
thropods. 

ARTHROPODS. 

We  all  know  the  intense  muscular  activity  of  ar- 
thropods,  especially    insects — the    arrowy   swiftness    of 


MUSCULAR    AND    SKELETAL   SYSTExMS. 


233 


the  flight  of  many  flies;  the  prodigious  leaps  of  fleas, 
three   hundred   times  their  own   length ;    the   enormous 
masses,  twenty  times  their  own  weight,  dragged  by  ants, 
etc. ;    and   yet  the  relation  of  muscle  to   skeleton,  and 
therefore  the  mechanism  of  motion  and  locomotion,  is 
wholly   different    from    that    of   vertebrates.     In    verte- 
brates we  have  an  internal  skeleton  and  the  muscles  act- 
ing on  it  from  the  outside;  in  the  case  of  arthropods  we 
have  an  external  skeleton  and  the  muscles  acting  on  it 
from  the  inside.     The  whole  animal  is  inclosed  in  a  skele- 
tal   coat   of  mail. 
The  body  is  a  hol- 
low, jointed  barrel    A 
inclosing  the  vis- 
cera, and  the  limbs 
are   hollow  pipes 
filled  with  muscle. 
The     manner     in 
which      this      ar- 
rangement is  used 
for  limb  motion  is 
shown  in  the  fol- 
lowing figures:  Fig.  149,  A  and  B,  represents  the  joints 
of  a  stovepipe  beveled  a  little  on  the  two  opposite  sides 
so  that  when  fitted,  the  one  in  the  other,  there  is  a  small 
vacant  space  between.     Now  if  the  interfitted  parts,  a  a, 
be  riveted  together,  and  strings,  ;//  ///,  within  the  pipe  be 
attached  to  the  beveled  margins,  we  have  a  perfect  hinge 
joint,  and  pulling  on  one  string  or  the  other  produces 
motion  in  two  directions  in  one  plane.     Now  the  mechan- 
ism for  limb  motion  in  all  arthropods  is  like  this,  except 
that  we  have  ligaments   instead   of   rivets,  and  muscle 
and  tendon  instead  of  strings  pulled  by  hand.     Fig.  150 
shows   four    joints  of    the    limb    of    a    crab   or   lobster 
and  the  manner  in  which  the  muscles  bend  the  limbs. 


Fig.  149. — Diagram  showing;  mode  of  action  of 
muscle  and  skeleton  in  an  arthropod  :  a,  the 
joint ;  w,  the  muscle. 


234   PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

It  is  evident  that  with  a  hollow  skeleton  and  muscles 
within,  only  hinge  joints  can  be  formed.  A  ball  and 
socket  is  impossible.  How,  then,  is  universal  motion 
effected  ?  This  is  done  by  two  hmge  joints  moving  in 
planes  at  right  angles  to  one  another,  as  in  the  diagram, 
Fig.  149.     If  we  examine  the  leg  of  any  arthropod,  say 


Fig.  150. — Four  joints  of  the  limb  of  a  crustacean  cut  so  as  to  show  the 
muscles  (mm)  and  their  attachments. 

a  lobster,  we  shall  find  that  the  consecutive  joints  are 
hinged  alternately  in  planes  at  right  angles  to  one 
another  (Fig.  150). 

Now  of  these  two  different  modes  of  relation  of 
skeleton  and  muscle,  which  is  the  best  ?  We  have  al- 
ready seen  the  intense  locomotive  activity  of  insects. 
Many  writers  hastily  conclude  that  the  nervous  and 
muscular  activity  of  insects  is  far  greater  than  that  of 
vertebrates,  or  else  that  their  mechanism  is  superior. 
This  is  probably  a  mistake.  The  superior  locomotive 
activity  of  insects  is  simply  the  result  oi  small  size.  It  is 
evident  that,  other  things  being  equal,  the  contractile 
power  of  a  muscle  varies  as  its  cross  section — i.  e.,  as  the 
square  of  its  diameter.  But  the  weight  to  be  moved — 
i.  e.,  the  weight  of  the  body — varies  as  the  cube  of  the 
diameter.  Therefore,  as  the  size  of  the  animal  in- 
creases, its  weight  increases  faster  than  its  muscular 
power.  Therefore  more  and  more  of  the  whole  energy 
is  used  up  for  support  of  weight,  and  less  and  less  is 
left  over  for  locomotion,  until,  if  the  animal  is  large 
enough,  the  whole  power  is  used  up  for  support,  and 
none  is  left  over  for  locomotion.     There  is  therefore  a 


MUSCULAR    AND   SKELETAL   SYSTEMS.  235 

limit  to  the  size  of  a  walking  animal,  and  a  much  lower 
limit  to  the  size  of  a  flying  animal.  Contrarily,  as  an 
animal  becomes  smaller,  the  muscular  energy  per  unit 
section  remaining  the  same,  the  weight  decreasing 
faster  than  the  power,  less  and  less  proportion  is  neces- 
sary for  support  of  weight,  and  more  and  more  is  left 
over  for  activity  of  all  kinds.  This  is  the  true  reason 
why  small  animals  seem  so  much  more  vivacious  than 
large  animals. 

WORMS. 

In  these  there  is  no  skeleton,  but  the  muscles  act 
directly  on  the  body  to  produce  motion  and  locomotion. 
We  have,  therefore,  in  these  an  entirely  different  mode 
of  action. 

Take  an  earthworm  as  a  good  example.  In  these 
there  are  two  kinds  of  muscular  fibers,  viz.,  the  longitu- 
dittal  3.n6.  the  )'ing  fibers.  The  longitudinal  fibers,  acting 
all  together,  shorten  the  body  ;  acting  on  one  side  or  an- 
other, bend  the  body  to  the  corresponding  side.  The 
ring  fibers  constrict  the  body,  and,  acting  all  together, 
elongate  it.  But  the  most  conspicuous  peculiarity  of 
all  the  fibers,  both  longitudinal  and  ring,  is  their //-<7/<z- 
gated  ?iCt\on.  Watch  an  earthworm  in  locomotion.  We 
observe  a  wave  of  constriction — i.  e.,  contraction  of  ring 
fibers — running  forward,  advancing  each  part  consecu- 
tively until  it  reaches  the  head,  which  then  advances  and 
takes  hold  ;  and  then  begins  a  wave  of  contraction  of 
longitudinal  fibers,  running  backward  and  bringing  up 
successively  the  parts  of  the  body  toward  the  head. 
Usually  several  such  waves  chase  each  other  along  the 
body.  This  kind  of  motion  is  so  characteristic  of  worms 
that  it  may  be  called  vermicular.  It  is  found  in  all  ani- 
mals, even  the  highest  vertebrates,  in  the  involuntary 
muscles,  especially  those  of  the  stomach  and  intestines. 


236  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

It  is   there  wholly  automatic.     It   is  probably  at   least 
semi-automatic  in  the  locomotion  of  worms. 

The  locomotion  of  all  wormlike  animals,  as,  for  ex- 
ample, caterpillars,  etc.,  is  similar  to  that  described. 

MOLLUSCA. 

Acephala. — In  oysters  there  is  only  one  conspicuous 
motion,  viz.,  that  of  closing  the  valves.  This  is  effected 
by  a  large  muscle  running  transversely  from  valve  to 
valve.  When  this  is  cut,  as  in  opening  an  oyster,  the 
valves  fall  apart.  The  purple  spot  seen  on  the  interior 
of  an  oyster  shell  is  the  place  of  attachment  of  this  mus- 
cle, and  the  firmer,  sometimes  tough,  portion  of  the 
oyster  is  the  muscle  itself.  When  the  muscle  relaxes, 
the  valves  open  by  means  of  an  elastic  substance  in  the 
hinge  of  the  valves;  when  it  contracts,  the  valves  are 
closed  with  great  force. 

In  clams  (Fig.  151)  there  are  two  of  these  transverse 
muscles  and  an   additional   locomotive  organ,  the  foot, 


Fig.  151. — Mactra  with  one  valve  removed,  showing  the  anterior  iattt)  and 
the  posterior  \pm)  shell  muscles,  and  the  foot  (/").     (From  Gegenbaur.) 

connected  with  the  two  transverse  or  shell  muscles. 
This  organ  contains  longitudinal  fibers  for  retraction, 
and  also  oblique  and  transverse  fibers  for  constriction 
and  consequent  protrusion.  It  is  used  for  locomotion 
and  also  for  burrowing  in  the  mud.  In  locomotion  it  is 
protruded,  takes  hold,  and  then  is  retracted,  and  thus 
draws  the  body  forward. 


MUSCULAR   AND    SKELETAL   SYSTEMS. 


237 


Fig.   152. — Snail  walking. 


Gastropoda. — In  snails  (Fig.  152)  and  slugs  the  loco- 
motive organ  ox  foot  is  an  elongated  flat  disk,  lying  along 
the  ground,  which  consists  of  muscular  fibers  running  in 
all  directions,  some  transverse,  some  oblique,  and  some 
longitudinal.  The  longitudinal  fibers  shorten,  the  ob- 
lique and  trans- 
verse constrict 
and  lengthen. 
A  snail  or  slug 
seems  to  glide 
slowly  and  con- 
tinuously along 
without  appar- 
ent mechanism. 
If  watched  care- 
fully, however, 
waves  of  con- 
striction and  ad- 
vance chase  one  another  from  one  end  of  the  foot  to 
the  other.  In  the  case  of  those  having  a  shell,  as  the 
snail,  muscular  fibers  from  the  foot  are  attached  to  the 
interior  of  the  shell,  whereby  the  foot  is  drawn  into  the 
shell.  Of  course  there  are  also  small  muscles,  ring  fibers, 
and  longitudinal  fibers  for  moving  the  tentacles. 

Cephalopoda. — Confining  ourselves  to  locomotion, 
we  find  here,  again,  a  new  kind.  The  squid  and  cuttle- 
fish are  invested  with  a  thick,  hollow,  muscular  mantle, 
within  which  are  inclosed  all  the  viscera,  but  with  con- 
siderable space  between  filled  with  sea  water.  Just  be- 
neath the  head  protrudes  a  conical  tube  [sipho/i),  valvu- 
larly  connected  with  the  hollow  in  such  wise  that  when 
the  mantle  contracts  the  water  is  forced  through  the 
siphon  with  great  force,  and  the  animal  is  shot  back- 
ward with  great  speed.  Besides  this  rapid  locomotion 
for  escape,  there  is  another  in  both  the  sc]uid  and  cuttle- 


238    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


fish,  but  different  in  the  two  cases.  The  squid  is  pro- 
vided with  a  horizontal,  arrowhead-shaped  caudal  fin 
(not  shown  in  the  diagram),  which  is   not   flapped  from 


Fig.  153. — Diagram  showing;  general  structure  of  a  squid  :  mc,  the  mantle 
cavity  ;  j,  the  siphon  ;  g,  gills ;  ib^  ink  bag ;  ish^  internal  shell ;  e^  eye ; 
oe,  oesophagus  ;  k,  kidney.     The  arrows  show  the  direction  of  currents. 

side  to  side,  as  in  fish,  nor  up  and  down,  in  the  man- 
ner of  whales,  but  locomotion  is  effected  by  waves  propa- 
gated   backward    or    forward — in   the  one   case   giving 

rise  to  gentle  forward,  in 
the  other  to  gentle  back- 
ward motion.  These  grace- 
\^-^      ful    movements    may    be 
7^   watched  in  an  aquarium. 
The  short-bodied  cut- 
tlefish,   on    the    contrary, 
uses  its  long,  flexible,  mus- 
cular arms  for  crawling  on 

Fig.  154.— Diagram  of  a  medusa  :  ««,      the    bottom,     or    even    for 
nerve ;  tn.  mouth  ;  st,  stomach  \  rt.         ,  •      ,  •  1 

radiating  tubes.  climbing  Up  rOCks. 


CCELENTERATA. 


Medusae. — As  we  are  taking  only  the  most  strik- 
ingly different  modes,  we  pass  over  the  echinoderms 
and  take  next  the  acalephs  or  jnedusce.     The  transparent 


MUSCULAR    AiMJ   SKELETAL    SYSTEMS. 


239 


character  and  the  saucerlike  (Fig.  154)  or  bell-shaped 
form  and  graceful  movements  of  these  beautiful  crea- 
tures are  well  known.  Their  locomotive  apparatus 
consists  of  fine  muscular  fibers,  arranged  in  circular 
and  radiating  manner  on  the  interior  of  the  saucer  or 
bell.  When  these  fibers  contract,  the  saucer  or  bell 
is  drawn  together,  the  water  expelled,  and  the  animal 
driven  in  the  contrary  direction.  When  the  fibers  relax, 
the  somewhat  firm,  gelatinous  mass  again  expands  by 
its  own  elasticity  to  its  original  form  and  size. 


PROTOZOA. 

Infusoria. — in  these,  again,  we  have  a  wholly  differ- 
ent mode,  VIZ.,  ciliary  Jiiotion  and  locomotion.  If  the  ani- 
mal be  fixed,  as  a,  then  the  incessant  lashing  of  micro- 
scopic cilia,  situated  mainly 
about  the  mouth,  creates 
whirling  currents,  which  lead 
down  to  the  throat,  and  thus 
contribute  to  alimentation. 
Whatever  is  suitable  as  food 
goes  down  ;  what  is  not,  is 
rejected  and  carried  away 
by  the  same  current.  But 
if  the  body  is  free,  b,  the 
same  ciliary  motion,  react- 
ing against  the  water,  produces  the  most  vivacious  loco- 
motion. Under  the  microscope  it  is  seen  to  whirl  and 
dart  about  in  every  direction  without  visible  means; 
but  with  the  higher  power,  especially  when  the  motion 
is  slower,  the  incessant  lashing  of  the  cilia  is  visible. 

Rhizopods. — Here    we    have    motion,   as  well    as    all 
other  functions,  reduced  to  simplest  terms,  and  find  its 
origin   in  general  contractility  of  protoplasm.     The  struc- 
tureless, or  almost  structureless,  mass  of  living  jelly  con- 
17 


Fig.    155. — Infusoria:    a,   an   at- 
tached form  ;  b,  a  free  form. 


240 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 


tracts  itself  in  any  part  and  in  any  direction,  putting  out 
here  or  there  a  fingerlike  projection  or  a  hairlike  thread, 

and  again  withdrawing  and 
absorbing  it  into  its  sub- 
stance (motion)  ;  or  such  a 
projection  may  take  hold  of 
some  movable  body,  and  then 


A  B 

Fig.  156. — Amoeba  proteus  (after  Leidy)  :  A,  with  the  pseudopods  ex- 
tended ;  B,  the  same,  with  pseudopods  withdrawn  ;  fb^  food  bodies ; 
ex,  excrement  discharged. 


by  retraction  the  movable  body  is  drawn  toward  and 
absorbed  as  food  into  the  animal  (prehension) ;  or,  final- 
ly, if  the  point  taken  hold  of  is  fixed,  then  the  creature 
is  moved  and  absorbed  into  the  fingerlike  projection 
(locomotion). 


CHAPTER    IV. 

GENERAL     LAWS     OF     ANIMAL     STRUCTURE,    OR     GENERAL 
LAWS  OF  MORPHOLOGY,   OR   PHILOSOPHICAL   ANATOMY. 

SECTION    I. 

Ititroductory. 

This  is  a  subject  of  fascinating  interest  in  many 
ways,  but  especially  on  account  of  its  bearing  on  the 
theory  of  the  origin  of  organic  forms  by  evolution.  It 
is  for  this  reason  mainly  that  we  shall  treat  it  some- 
what fully. 

I  find  it  necessary,  however,  to  make  some  prelimi- 
nary definitions  of  terms. 

Analogy  versus  Homology. — Parts  or  organs  in 
different  animals  are  said  to  be  analogous  when  they 
have  a  similar  form,  and  especially  when  they  perform 
a  similar  function.  Contrarily,  parts  or  organs  of  dif- 
ferent animals  are  said  to  be  homologous  when,  however 
different  their  general  apearance  and  however  different 
their  functions,  they  can  be  shown  to  have  a  common 
origin — to  be,  in  fact,  the  same  part,  only  modified  in 
order  to  perform  different  functions.  The  difference 
between  these  can  be  best  shown  by  examples ;  and  the 
ideas  involved  in  these  terms  lie  at  the  basis  of  all  I 
shall  say.  We  will  give  examples  from  plants,  as  well 
as  animals. 

Examples :  Animals. — i.  The  7C'ing  of  a  bird  and  the 
wing  of  a  butterfly  are  analogous  organs.  They  have  a 
similar  flat   form,  adapting  to   a  similar  function — viz., 

241 


242 


PHYSIOLOGY   AND    MORPHOLOGY   OF  ANIMALS. 


that  of  flying.  But  they  are  not  at  all  homologous. 
They  have  not  a  common  origin ;  the  one  could  not 
have,  and  did  not,  come  from  the  other.  In  a  word, 
they  are  not  at  all  the  same  thing.  But  the  wing  of  a 
bird  and  the  wing  of  a  bat  are  homologous  parts.  Not 
only  so,  but  both  are  homologous  with  the  pectoral  fin 
of  a  fish,  the  fore  limb  of  a  reptile  or  a  mammal,  and 
the  arm  and  hand  of  a  man  ;  for  all  these,  though  now 
so  different  in  form  and  function,  can  be  shown  to  have 
a  common  origin  by  descent — to  be,  in  fact,  the  same 
thing,  only  modified  for  various  purposes. 

2.  The  /u»gs  of  man  and  the  gills  of  a  fish  are  analo- 
gous parts — i.  e.,  they  perform  the  same  function  in  the 
animal  economy — viz.,  the  aeration  of  the  blood.  But  they 
are  not  homologous.  By  no  possibility  could  one  have 
come  by  modification  out  of  the  other.  What,  then,  in 
the  fish  is  the  organ  homologous  with  the  lung  of  man  ? 
It  is  the  air  bladder.  This  is  the  organ  which  by  modifi- 
cation became  the  lung  of  air-breathing  animals.  The 
proof  of  this  is  complete,  for  all  the  steps  of  the  gradual 
change  can  be  traced.     This  is  shown  as  follows : 

{a)  In  most  typical  fishes  the  air  bladder  is  wholly 
isolated  and  used  only  as  a  float.  In  such  cases  it  is 
colorless.  But  in  some  fishes  it  is  connected  by  a  slen- 
der tube  with  the  throat,  and  doubtless  the  contained 
air  is  renewed  from  that  source.  In  still  other  fishes  it 
is  not  only  connected  with  the  throat,  but  air  is  regu- 
larly and  voluntarily  taken  in.  In  such  cases  it  is  vas- 
cular and  therefore  reddish  in  color.  This  is  the  case  in 
the  gar  pike  [Lepidosteiis).  This  fish  may  be  observed 
to  come  at  intervals  to  the  surface  and  gulp  down  air, 
which  is  again  afterward  expelled  in  bubbles.  The  gill 
breathing  is  to  some  extent  supplemented  by  air  breath- 
ing. Finally,  in  a  few  of  the  most  reptilian  fishes  [Cerato- 
diis  and  lepidosiren)  the  air  bladder  is  not  only  vascular, 


GENERAL   LAWS   OF   ANIMAL   STRUCTURE.      243 

but  cellular,  like  the  lung  of  a  frog.  It  is  really  a  lung, 
and  these  animals  breathe  partly  by  gills  as  a  fish,  and 
partly  by  lungs  as  an  amphibian  reptile.  It  is  an  am- 
phibian fish. 

[^)  Now,  the  condition  last  described  is  exactly  that 
of  the  lowest  amphibians  and  the  early  or  larval  condi- 
tion of  a//  amphibians.  In  fact,  in  the  development  of 
the  individual,  in  the  individual  life  history  of  an  am- 
phibian, we  have  all  the  stages  of  change  of  the  respira- 
tory organs  from  the  complete  gill-breathing  stage  to  the 
complete  lung-breathing  stage.  The  very  young  tad- 
pole breathes  wholly  by  gills,  like  a  fish  ;  the  frog  wholly 
bv  lungs,  like  a  reptile  or  a  mammal.  How  did  the 
change  take  place?  By  modification  of  the  gills?  No 
— but  dy  the  development  of  another  organ  in  the  position  of 
the  air  bladder  of  a  fish.  At  first,  as  already  said,  the 
breathing  is  wholly  by  gills,  like  a  fish.  Then  by  the  de- 
velopment of  an  organ  in  the  position  of  the  air  bladder 
of  the  fish  and  connected  with  the  throat  it  begins  to 
breathe  also  by  a  commencing  lung.  It  now  breathes 
equally  by  gills  and  by  lungs,  both  water  and  air.  This 
is  the  permanent  condition  of  the  highest  fishes  (lung 
fishes)  and  of  the  lowest  amphibians  (the  Ferenni- 
branchiata),  such  as  the  siren,  etc.  From  this  time  onward 
the  gills  decrease  and  the  lungs  increase,  until  in  the 
adult  the  gills  disappear  and  the  breathing  is  wholly  by 
lungs.  The  first  argument  [a)  is  strongly  presumptive; 
the  second  [b)  is  demonstrative. 

It  is  unnecessary  to  give  further  examples  from  ani- 
mals, as  all  that  we  shall  have  to  say  in  this  chapter  will 
be  a  continuous  illustration  of  homology  ;  but  a  few 
illustrations  from  plants  will  show  the  universality  of  the 
principle. 

(i)  A  rhizome  or  rootstock — such  as  that  of  a  flag, 
or  of  calamus,  or  of  a  fern — is  analogous  to  a  root,   for 


244   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

it  has  the  underground  position  of  a  root,  and  probably 
performs  some  of  the  functions  of  a  root.  But  it  is 
homologous  with  a  stem — it  is,  in  fact,  an  underground 
stem,  for  it  has  the  structure  of  a  stem,  it  bears  leaves 
like  a  stem,  and  in  the  axils  of  those  leaves  come  buds, 
making  new  shoots.  Moreover,  in  many  plants,  as  in 
palms  and  in  ferns,  all  gradations  can  be  traced  even 
in  the  same  family  from  the  underground,  through  the 
prostrate  and  inclined,  to  the  upright  position. 

(2)  The  so-called  leaf  of  a  cactus  is  no  doubt  analogous 
to  a  leaf.  It  is  flat  and  green,  and  performs  the  func- 
tions of  a  leaf — viz.,  the  assimilation  of  plant  food.  But 
it  is  homologous  with  a  stem.  It  is,  in  fact,  a  stem  modi- 
fied in  shape  and  color  to  perform  the  function  usually 
performed  by  leaves.  This  is  proved  by  its  structure — 
viz.,  pith,  wood,  and  bark,  with  medullary  rays  connect- 
ing the  pith  and  bark ;  and  also  by  the  gradations 
among  cactuses  between  the  flat,  leaflike  form  and  the 
cylindrical,  stemlike  form.  Where,  then,  are  the  true 
homologues  of  the  leaves  ?  We  find  them  in  the  spines. 
These  are  abortive  leaves  modified  as  defensive  organs. 
They  have  the  spiral  arrangement  of  leaves,  and  in 
their  axils  come  the  buds  which  form  new  shoots. 

3.  One  more  example  :  The  acacias — of  which  there 
are  in  California  about  twenty  species,  introduced  from 
Australia — are  by  appearance  easily  divided  into  two 
groups,  viz.,  the  feathcr-lcaved  acacias  and  the  j-/;«//^- 
/?d!Z^^^ acacias.  These  are  so  different  in  general  appear- 
ance, that  the  mere  popular  observer  would  probably 
put  them  not  only  in  different  genera,  but  even  in  differ- 
ent families.  But  doubtless  the  botanists  are  right  in 
putting  them  all  in  the  same  genus,  for  they  are  really 
so  closely  allied  that  the  same  individual  may  pass  from 
one  form  to  the  other. 

The  fact  is,  the  so-called   leaf  of  the  simple-leaved 


GENERAL    LAWS   OF   ANIMAL    STRUCTURE. 


245 


acacias  is  not  homologically  a  leaf  at  all,  but  a  leaf- 
stem,  broadened  and  flattened  to  perform  the  function 
of  a  leaf.  Such  an  organ,  functionally  a  leaf  but  homolo- 
gous with  a  leaf-stem,  is  called  by  botanists  a  phyllode. 
Its  true  nature  is  shown  in  the  development  of  a  simple- 


FlG.  157. — K  branch  of  young;  acacia,  showing  change  from  one  form  of 
leaf  to  the  other  :  <7,  ^,  c,  d^  .successive  stages  of  change  ;  /y,  leaf  stalk 
which  gradually  changes  into  the  blade  in  c,  d,  and  e. 

leaved  acacia  from  a  seedling.  At  first  it  bears  only 
feather  leaves,  then  imperfect  feather  leaves  with  flat- 
tened stems,  then  broadly  flattened  stems  with  a  mere 
remnant  of  featherlike  leaflets,  and  finally  the  leaflets 
are  gone  and  only  the  broad  leaf-stems  remain.  All  these 
forms  may  often  be  seen  in  the  same  spray  (Fig.  157). 


246   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

After  these  illustrations,  we  come  back  to  define 
again  these  terms. 

Analogy  is  founded  on  fu?iction.  Homology  on  com- 
moti  origin  by  descent.  Parts  of  most  diverse  origin 
may  be  modified  to  perform  the  same  function,  and 
therefore  assume  a  general  resemblance.  This  is  anal- 
ogy. Parts  of  the  same  origin — really  the  same  parts  in 
different  species — by  modification  for  different  functions 
may  become  so  different  that  they  are  no  longer  easily 
recognized  as  the  same  part.  This  is  homology.  In 
the  one  case  the  parts  seem  like  the  same  and  behave 
like  the  same,  but  are  really  very  different ;  in  the  other 
they  are  really  the  same  part,  but  they  seem  to  the  su- 
perficial observer  to  be  very  different  both  in  appear- 
ance and  in  behavior.  In  the  one  case  there  is  a  superfi- 
cial resemblance  produced  by  functions  easily  observed, 
and  therefore  determining  popular  names;  in  the  other 
there  is  a  deep-seated  resemblance  shown  by  essential 
structure  and  structural  relations,  but  more  or  less  ob- 
scured by  adaptation  to  different  functions.  This  deep- 
seated  resemblance  can  only  be  found  by  wide  com- 
parison in  the  animal  series  and  in  the  embryonic  series, 
for  it  is  in  this  way  only  that  we  find  the  steps  of  grada- 
tion connecting. 

There  are  therefore  two  ideas  underlying  homology, 
viz.,  common  descent  and  adaptive  modificatio7i.  Things 
having  common  origin  by  descent  may  be  so  modified 
to  adapt  them  to  different  functions  as  to  conceal  their 
common  origin.  It  is  the  duty  of  the  morphologist  to 
trace  out  the  evidences  of  common  descent  in  spite  of  the 
obscurations  of  adaptive  modification. 

The  idea  of  homology  or  common  origin  with  obscura- 
tions by  adaptive  modification,  lies  at  the  very  basis  of 
biology  and  must  be  universal.  We  have  therefore  an  ex- 
cellent e.xample  of  it  in   essential  cell  structure  charac- 


GENERAL   LAWS   OF   ANIMAL   STRUCTURE.      247 

teristic  of  all  living  things.  We  have  already  given  this 
illustration  on  page  22.  All  tissues  have  a  common 
origin  in  unmodified  cell  structure.  But  in  the  differ- 
entiated tissues  of  the  higher  animals  this  common 
origin  is  so  obscured  by  adaptive  modification  for  func- 
tion that  it  can  not  be  made  out  except  by  extensive 
comparison  in  the  animal  scale  and  in  the  embryonic 
scale.  Now  the  object  of  this  chapter  is  to  trace  the 
homologies  or  the  common  origin  in  animal  structure  in 
spite  of  the  obscurations  produced  by  modification  of 
parts  adapting  it  to  various  functions^  and  of  the  whole 
organism  adapting  it  to  various  places  in  the  economy  of 
Nature. 

But  the  question  occurs :  How  far  can  we  trace 
homology  ?  Analogy  can  be  traced  throughout  the  ani- 
mal kingdom,  because  it  is  based  on  function.  Doubt- 
less, also,  homology  or  common  descent  must  exist 
throughout,  but  it  is  not  traceable  with  certainty.  The 
obscurations  of  adaptive  modification  completely  oblit- 
erate the  evidences  of  common  origin  except  in  animals 
not  too  far  separated  in  character.  How  far,  then,  can 
we  satisfactorily  trace  it?  ^^'e  answer:  Only  through 
the  primary  divisions  or  departments.  And,  conversely 
from  the  morphological  or  evolutional  point  of  view,  this 
ability  to  trace  common  origin  is  the  true  ground  of  pri- 
mary divisions.  From  this  point  of  view  Agassiz's  four 
primary  branches  from  the  protozoan  trunk  finds  much 
justification. 

Thus,  then,  we  would  say,  the  structure  of  all  7'erte- 
brates  is  exactly  what  it  would  be  if  they  all  came  by 
descent  with  modification  from  one  primal  vertebrate; 
and  therefore  we  must  believe  they  did  so  come.  All 
articulates — i.  e.,  arthropods  and  annulates — for  like  rea- 
son, came  from  a  primal  form  of  articulate  by  adaptive 
modification.     Similarlv  all   mollusca  came   bv  descent 


248    THYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

with  modification  from  some  primal  form  of  mollusk; 
and  radiates  (including  echinoderms  and  ccelenterates) 
from  some  primal  form  of  radiate  animal. 

That    these    prime    divisions    also    have    a    common 
origin  is  shown  by  their  common  cellular  structure,  and 

also  by  the  fact  that  the  first 
steps  in  embryonic  devel- 
opment is  the  same  in  all. 
But  this  common  origin  is 
not  shown  in  the  completed 
structure. 

From  this  point  of  view, 
at  least  four  main  branches 
of  the  tree  of  life  came  from 
t\\^  protozoan  trunk  and  grew 
in  different  directions,  di- 
verging more  and  more,  and 
giving  off  each  subordinate 
branches  (classes,  orders,  families,  etc.) ;  but  the  point  of 
union  with  the  protozoan  trunk  and  with  one  another  is, 
as  it  were,  hidden  from  view  beneath  the  common  ground 
of  cellular  structure.  But  the  subordinate  branches  of 
each  primary  branch  can  be  traced  throughout.  Dia- 
gram Fig.  158  roughly  expresses  what  we  mean. 


Fig.  158.^ — Diagram  showing  sup- 
posed common  origin  of  the  dif- 
ferent phyla. 


SECTION    II. 

Homology  of  Vertebrates. 

The  thesis  to  be  established  is  that  all  vertebrates 
have  come  from  some  primal  form  of  vertebrate  by 
modification.  This  is  best  shown  in  the  skeleton,  and 
we  shall  confine  ourselves  mainly,  though  not  entirely, 
to  this  system.  The  common  origin  is  shown,  first, 
in  the 


GENERAL    LAWS   OF   ANIMAL   STRUCTURE.      249 


ns 

vert 

b 


I.    GENERAL    PLAN    OF    STRUCTURE GENERAL 

HOMOLOGY. 

1.  All  vertebrates,  and  no  other  animals,  have  an 
interior  skeleton,  with  the  muscles  acting  on  it  from  the 
outside  to  produce  motion  and  locomotion. 

2.  In  all  vertebrates,  and  in  no  other  animals,  the 
axis  of  this  skeleton  is  a  jointed  backbone  or  vertebral 
column.  It  is  this  that  gives  name  to  this  department. 
They  are  vertebrates  or  backboned  animals. 

3.  In  all  vertebrates,  and  in  no  other,  this  axis  in- 
closes two  cavities  :  one,  the  neural  cavity,  //,  is  above,  to 
protect  the  nervous  centers; 
and  the  other,  the  visceral  cav- 
ity, ?',  below  to  inclose  the  vis- 
ceral centers  (Fig.  159). 

4.  In  all  vertebrates,  and 
in  no  other,  the  head  may  be 
regarded  as  a  coalescence  of 
several  vertebral  joints.* 

5.  In  nearly  all  vertebrates, 
and  in  no  other,  there  are 
two,  and  only  two,  pairs  of 
limbs.  The  exceptions  to  this 
law  fall  under  two  categories 
— viz.,  those  like  the  lowest 
fishes,  which  represent  the 
condition  before  limbs  were 
added  to  the  skeleton,  and 
those  like  snakes  and  like  some  lizards  and  amphibians, 
in  which  the  limbs  have  been  lost. 

This  general  common  plan  of  skeletal  structure 
strongly  suggests  a  common  origin. 


Fig.  159. — Cross  section  through 
a  fish  :  vs,  visceral  system  ; 
verf,  vertebra ;  b,  blood  sys- 
tem ;  ?is,  nervous  system. 


*  Some  do  not  hold  this  view  of  the  origin  of  the  head. 


250 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


II.    SPECIAL    HOMOLOGY. 

But  not  only  do  we  find  a  common  general  plan  sug- 
gesting common  origin,  but,  when  examined  thought- 
fully, we  find  all  vertebrate  skeletons  exactly  corre- 
sponding, part  for  part,  bone  for  bone,  only  modified  by 
necessary  adaptation  for  various  functions.  The  modi- 
fications may  be  so  great  as  to  obscure  the  essential 
identity,  but  extensive  study  of  comparative  anatomy 
and  embryology  reveal  the  missing  links  in  the  continu- 
ous chain  of  change.  The  evidence  here  is  demon- 
strative. 

Limbs. — To  show  this  we  take  the  case  of  the 
limbs,  partly  because  their  structure  is  better  known  to 
common  observation,  but  mainly  because  we  have  in 
them  the  two  ideas  of  common  origin  and  adaptive 
modification,  both  equally  illustrated.  The  modifica- 
tions here  have  been  great,  but  not  so  great  as  to 
wholly  conceal  the  common  origin. 

(a)  Fore  Limbs. — For  fore  limbs  we  take  man's  as 
the  type  or  term  of  comparison  because  it  is  best  known, 
and  also  because  it  is  really  far  less  modified  than  many 
others;  although  in  this  regard  alone  that  of  reptiles 
would  be  the  best. 

See,  then,  the  fore  limbs  of  various  classes  of  verte- 
brates (Figs.  160,  161,  162).  The  same  parts  are  similarly 
lettered  in  all,  so  that  the  legend  sufficiently  explains 
the  corresponding  parts.  Moreover,  dotted  lines  are 
drawn  through  the  most  important  corresponding  parts 
to  make  the  comparison  more  easy.  But  it  is  necessary 
to  say  something  more  in  the  way  of  explanation 
concerning  several  of  these  parts. 

I.  Shoulder  Girdle. — This  consists  of  three  important 
bones — viz.,  the  blade  (scapula),  the  clavicle,  and  the  cora- 
coid.     The  type  is  seen  in  the  reptile  (Fig.  162,  A)  which 


GENERAL  LAWS  OF  ANLMAL  STRUCTURE. 


251 


may  be  regarded  as  the  original  form  of  the  land  animal. 
In  these  all  the  parts  are  large.  The  coracoid  is  as  large 
as  the  blade,  and  is  joined  firmly  to  the  sternum,  making 


3    V 

rt  3 
«  o 


tJ5  c 

O   4) 


..  o 
E  u, 

P  ? 


<  -v 


thus  a  firm  girdle  for  the  attachment  of  the  fore  limb. 
In  birds  also  the  coracoid  is  a  large  bone  firmly  united 
to  the  breastbone  (sternum).  These  are  types  of  shoul- 
der girdle.     In  man  the  coracoid  is  reduced  to  a  small 


252 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


process  of  the  blade.  In  most  mammals  the  coracoid  is 
still  more  reduced,  and  the  clavicle  is  wanting  alto- 
gether ;  but  all  the  steps  of  its  gradual  obsolescence 
may  be  picked  up  by  extensive  comparison. 

2.  Elbow  Joint. — In  man,  in  monkeys,  and  in  some 
other  mammals,  and  in  all  reptiles,  the  whole  limb  is 
free  of  the  body  and  the  elbow  joint  halfway  down  the 
limb.  This  is  undoubtedly  the  original  and  typical  con- 
dition, but  in  all  highly  specialized  mammals,  by  the 
shortening  of  the   humerus,  the  elbow  is  drawn  up  on 


Fig.  161. — A,  fore  limb  of  bat ;  B,  bird  ;  C,  archasopteryx  ;  D,  pterodactyl. 
(Lettered  as  in  previous  figure  ;  grouped  from  various  sources. ) 

the  side  of  the  body  so  that  the  limb  is  free  only  from 
the  elbow  downward. 

3.  In  man,  in  monkeys,  and  many  other  mammals, 
and  in  all  birds  and  reptiles,  there  are  two  bones  in  the 
forearm.  But  in  all  the  more  specialized  mammals 
these  are  reduced  to  one.,  although  the  remnant  of  the 
other  is  always  found  forming   the  point  of  the   elbow 


GENERAL   LAWS   OF  ANIMAL   STRUCTURE.      253 

(see  figure  of  horse,  Fig.  160,  E).     All  the  gradations  are 
easily  found. 

4.    JFrist  Joint. — In  man  (when  he  comes  down  on  all 
fours),  in  monkeys,  in  bears,  and  some  other  mammals, 


A  BCD 

Fig.  162. — A,  fore  limb  of  turtle  ;  B,  mole  ;  C,  whale  ;  D,  fish. 

and  in  all  reptiles.,  the  wrist  joint  comes  down  to  the 
ground — the  tread  is  on  the  whole  hand.  This  was  the 
original  tread  of  early  land  animals;  but  in  all  the 
more  specialized  mammals,  and  especially  in  hoofed  ani- 
mals, the  tread  is  on  the  toes,  and  the  wrist  is  elevated 
far  above  the  ground,  and  in  hoofed  animals,  as  the 
horse  and  the  cow,  is  usually,  but  erroneously,  called 
the  knee. 

5.  In  man,  in  many  mammals,  and  in  all  reptiles, 
there  are  five  palm  bones  (metacarpals)  and  five  fingers, 
and  this  was  undoubtedly  the  original  and  typical  num- 
ber ;  but  in  all  the  more  specialized  mammals  the  num- 
ber of  toes  is  reduced  in  some,  as  the  carnivores,  to 
four,  in  some  hoofed  animals  to  three  (rhinoceros),  in 
some  to  two,  as  in  ruminants,  and  in  the  horse  to  one. 
The  palm  bones  in  ruminants  are  reduced  to  one  (canon 


254 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


bone)  and  in  the  horse  also  to  one,  but  with  two  rudi- 
ments of  others  (splints).  But  in  all  cases  we  can  trace 
all  the  steps  of  gradation.  This  is  well  shown  in  rumi- 
nants. In  the  deer,  besides  the  two  functional  toes, 
there  are  two  rudiments — hoofed  and  supplied  with 
bones — but  useless;  in  the  cow  these  are  further  re- 
duced to  mere  wartlike  rudiments;  in  the  goat  and 
sheep  they  disappear  entirely. 

6.  We  need  hardly  call  attention  to  the  extreme 
modification  of  the  hand  in  the  bird's  wing,  of  the  whole 
fore  limb  in  the  whale  and  in  the  mole,  and  especially  in 
this  last  to  the  elongation  of  the  point  of  the  elbow  to 
increase  its  power  of  digging  (Fig.  162,  B)  and  to  the 
still  more  extreme  modification  in  the  case  of  the  pec- 
toral fin  of  fishes,  and  the  enormous  elongation  of  one 
finger  of  the  hand  in  the  extinct  flying  reptile — the 
pterosaurs  of  the  Mesozoic  (Fig.  161,  D). 

It  is  well  to  observe,  too,  how  the  same  part  may  be 
differently  modified  for  the  same  function.  This  is  well 
illustrated  by  the  different  devices  used  for  flight  in 
birds,  in  mammals  (bat),  and  in  reptiles  (pterodactyls).  In 
the  bird  the  hand  is  shortened  and  consolidated,  and  the 
flat  plane  is  formed  by  the  addition  of  quill  feathers.  In 
the  bat  the  hand  is  elongated,  and  a  web  is  stretched  be- 
tween the  greatly  lengthened  palm  bones  and  fingers, 
leaving  only  one  finger,  the  thumb,  free  and  clawed.  In 
the  flying  reptile  one  finger  is  enormously  elongated  and 
strengthened,  and  the  web  is  stretched  from  the  point  of 
this  to  the  hind  limb,  leaving  the  three  other  fingers 
free  and  clawed.  It  is  a  most  significant  fact,  however, 
that  these  several  devices  are  not  accomplished  at  once, 
but  by  a  gradual  process  through  successive  generations. 
In  the  earliest  bird  (the  Archaopteryx)  (Fig.  161,  C)  of 
the  Jurassic  the  finger  bones  are  not  consolidated  nor 
feathered,  but  are  all  four  free  and  clawed. 


GENERAL  LAWS  OE  ANIMAL  STRUCTURE.   255 


{/>)  Hind  Limbs. —  1  lie  hind  limbs  are  less  specialized 
than  the  fore  limbs.     We  have  therefore  contented  our- 


selves with  illustrations  from  mammals   alone,  as  thefee 

are  the  most  specialized  of  vertebrates  (Fig.  163,  A,  B, 

C,  D,  E).     We  have  again  drawn  dotted  lines  through 
i3 


256    PHYSIOLOGY   AND    MORPHOLOGY    OF    ANLMALS. 

the  important  corresponding   parts.      We  draw  special 
attention  to  the  following  points: 

1.  Hip  Girdle. — This  is  particularly  strong  in  all  bi- 
peds, and  therefore  in  man,  in  birds,  in  kangaroos,  and 
in  the  extinct  biped  reptiles  (dinosaurs).  The  hip 
girdles  are  not  represented  in  the  figure. 

2.  Position  of  the  Knee. — In  man,  in  monkeys,  and 
bears  among  mammals,  and  in  all  reptiles  and  amphibians, 
the  whole  hind  limb  is  free  of  the  body  and  the  knee  is 
halfway  down  the  limb.  This  is  undoubtedly  the  origi- 
nal and  normal  condition  in  land  vertebrates ;  but  in 
the  more  specialized  mammals,  such  as  carnivores  and 
herbivores,  especially  the  ungulates  or  hoofed  animals, 
the  knee  is  high  up  on  the  side  of  the  body,  in  the  mid- 
dle of  the  so-called  thigh,  and  the  limb  is  free  of  the  body 
only  from  the  knee  down. 

3.  Position  of  the  Heel. — -In  man,  in  monkeys,  in  the 
bear,  and  several  other  mammals,  and  in  all  reptiles  and 
amphibians,  the  tread  is  on  the  whole  foot — i.  e.,  heel 
down.  This  is  undoubtedly  the  original  and  normal 
tread  of  the  primal  land  vertebrate.  But  in  all  the  more 
specialized  mammals  the  heel  is  lifted  high  in  the  air — 
in  the  horse  fifteen  to  eighteen  inches  above  ground — 
and  the  tread  is  on  the  toes  only.  Therefore  land  verte- 
brates, as  to  their  tread,  may  be  divided  into  two 
groups,  \\z.,  platttigrade  and  digitigrade.  Man,  monkeys, 
bears,  and  some  other  mammals,  and  all  reptiles  and 
amphibians,  are  plantigrade,  but  all  the  more  specialized 
and  swifter  mammals  and  all  birds  are  digitigrade — 
tread  only  on  their  toes. 

Again,  in  mammals  there  are  two  degrees  of  digiti- 
gradeness.  The  carnivores  and  all  other  clawed  digiti- 
grades  and  all  birds  tread  on  the  whole  length  of  the 
toes  to  the  ball,  while  the  hoofed  mammals  (ungulates) 
tread  only  on  the  tip  of  the  last  joint  of  the  toes.     The 


GENERAL    LAWS   OF   ANLMAL    STRUCTURE.      257 

one  treads  tiptoe,  the  other  on  the  tip  of  the  toe. 
These  may  be  called  unguligrade.  This  becomes  the 
more  striking  when,  as  in  the  horse,  the  tread  is  on  the 
tip  of  the  last  joint  of  the  one  toe.  We  look  with  won- 
der and  delight  at  the  danseuse  pirouetting  on  the  tip  of 
one  toe.     The  horse  is  doing  this  all  the  time. 

4.  Maims  and  Pes. — There  are  two  senses  in  which 
we  may  use  the  term  foot,  (i)  We  may  use  it  as  that 
part  on  which  the  animal  treads.  This  is  the  functional 
or  analogical  sense.  Or  (2)  we  may  use  it  as  that  part 
which   corresponds   to    the   foot  of  man,  monkey,  bear, 


Fig.   164. — Restoration  of  Rhamphorhynchus  phyllurus  (after  Marsh). 
One  seventh  natural  size. 

reptile — the  original  foot.  This  is  the  morphological  or 
homological  sense.  It  is  in  this  latter  sense  that  we  use 
it  in  comparative  anatomy.  In  this  sense  the  horse's 
foot  is  eighteen  inches  long,  and  the  hand  of  the  largest 
flying  dragon  [Fteranodon  ingens)  was  fully  seven  or  eight 
feet  long  (Fig.  164).  In  order,  however,  not  to  violate  too 
flagrantly  common  usage,  comparative  anatomists  use  the 
Latin  terms  manus  d,x\di  pes  to  signify  all  that  corresponds 
to  the  hand  and  foot  of  man  and  plantigrade  animals. 

5.   Classification  of  Ungulates  by  Foot  Structure. — The 
ungulates  or  hoofed  animals  are  divided  by  foot  struc- 


258   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

ture  into  two  groups,  viz.,  artiodactyle  or  even-toed,  and 
perissodactyle  or  odd-toed.  The  artiodactyles  may  have 
four  toes,  as  in  the  hippopotamus  and  the  hog,  or  two 
toes,  as  in  ruminants.  The  perissodactyles  may  have 
three  toes,  as  in  the  rhinoceros  and  tapir,  or  one  toe  only, 
as  in  the  horse  and  zebra.  Both  of  these  groups  have 
come  from  a  five-toed  ancestor  by  successive  dropping 
of  toes.  There  is  also  a  regular  order  in  the  dropping. 
In  the  five-toed  ancestor  the  two  groups  were  not  yet 
differentiated.  The  first  toe  to  drop  was  the  thumb, 
leaving  the  remaining  four  toes  all  functional,  as  in  the 
hippopotamus.  Then,  if  evolution  is  on  the  artiodac- 
tyle line,  the  two  side  toes  gradually  dwindle  and  shorten 
up,  as  in  the  hog,  and  finally  disappear,  as  in  ruminants, 
leaving  only  two  greatly  enlarged  toes,  which  correspond 
to  the  middle  and  ring  fingers  of  man.  But  if,  on  the 
other  hand,  the  animal  is  on  the  perissodactyle  line, 
then,  of  the  four  toes,  the  little  finger  first  dwindles,  as 
in  the  fore  foot  of  the  tapir,  and  then  disappears,  leaving 
three.  Now,  of  these  three,  the  side  toes  dwindle  and 
shorten  up  and  finally  disappear,  leaving  only  the  great- 
ly enlarged  middle  toe,  as  in  the  horse. 

6.  Rudimentary  Organs  ;  Useless  Orgatis. — All  through 
the  animal  kingdom,  especially  in  the  more  specialized 
forms  of  mammals,  we  find  rudimentary  and  often 
wholly  useless  organs.  These  are  evidently  remnants 
of  once  useful  organs,  which  have  dwindled  by  disuse, 
but  have  not  yet  entirely  disappeared.  Examples  meet  us 
on  every  side,  (a)  The  horse's  toes  are  reduced  to  one,  but 
not  at  once.  It  was  a  gradual  process,  every  step  of  which 
may  be  traced.  The  first  representative  of  the  horse 
in  the  line  of  its  descent  had  five  toes  on  the  fore  foot, 
then,  in  later  times,  four,  then  three.  Three-toed  horses 
continued  a  long  time,  the  two  side  toes  meanwhile 
dwindling  and  shortening  up  and   finally  passing  away. 


GENERAL    LAWS    OF   ANL\LVL    STRUCTURE.      259 
b  e  d  e  J  9 


Equus :  Qua- 
ternary and 
Recent. 


Orohippus  : 
Eocene. 


Fig.  i6";.— Diagram  illustrating  gradual  changes  in  the  horse  family  : 
Throughout  a  is  fore  foot ;  t>,  hind  foot  ;  c,  forearm  ;  d,  shank  ;  e,  molar 
on  side  view  ;  /  and  g,  grinding  suiface  of  upper  and  lower  molars. 
(After  Marsh.) 


26o   PHYSIOLOGY  AND    MORPHOLOGY   OF    ANLMALS. 

But  the  two  palm  bones  to  which  they  were  attached 
still  remain  as  useless  splints  to  attest  the  original 
three-toed  condition.  These  and  other  changes  are 
shown  in  diagram,  Fig.  165.  (d)  In  ruminants  the  toes 
are  reduced  to  two,  but  we  have  already  shown  (page 
253)  that  useless  remnants  of  the  other  two  are  still 
found  in  most  ruminants,  showing  their  original  four-toed 
condition,  (c)  Dewclaws  in  the  dog  and  many  other 
mammals  reveal  an  original  five-toed  condition,  (d)  The 
whale  is  a  very  specialized  mammal,  and  therefore  full  of 
useless  remnants.  Whales  have  no  hair,  but  rudiments  of 
hair  in  the  skin  show  that  their  ancestors  were  hairy. 
They  have  no  hind  limbs,  but  rudiments  are  found  buried 
in  the  flesh,  and  therefore  useless.  The  baleen  whales 
have  no  teeth,  but  rudiments  of  teeth  are  found  buried 
in  the  jawbone,  and  are  never  cut.  (e)  We  have  already 
said  (page  249)  that  snakes  have  lost  their  limbs  by  disuse. 
This  is  true,  for  rudiments  of  limbs  are  found  buried  in  the 
flesh  of  some  (the  python),  and,  of  course,  useless.  (/) 
Even  in  man,  although  he  is  far  less  specialized  than  most 
mammals,  rudiments  are  found.  Among  these  may  be 
mentioned  the  muscles  for  moving  the  ears  and  for  mov- 
ing the  scalp.  Rudiments  must  be  regarded  as  demon- 
strative  proof  of  the  derivative  origin  of  organic  forms. 

Whole  Skeleton. — We  have  taken  only  the  limbs 
as  best  illustrating  the  principle,  but  the  same  is  true  of 
the  whole  skeleton.  The  skeleton,  as  a  whole,  is  in  all 
vertebrates  the  same  machine,  but  modified  in  all  its  parts 
to  adapt  it  for  various  purposes — viz.,  as  a  swimming 
machine,  a  walking  and  running  machine,  or  a  flying  ma- 
chine, and  all  without  essential  change  of  plan. 

Other  Systems. — The  whole  argument  for  deriva- 
tive origin  of  organic  forms  is  based  on  the  equal  bal- 
ance of  the  two  ideas — essential  identity  and  adaptive 
modification.     If  the  modification  be  too  great,  the  essen- 


GENERAL   LAWS   OF  ANIMAL   STRUCTURE.      261 

tial  identity  and  derivative  origin  of  parts  may  be  ob- 
scured or  even  obliterated.  If,  on  the  other  hand,  it  be 
too  small,  then  the  identity  may  have  come  in  some 
other  way  than  by  common  origin — may  have  been  made 
out  of  hand  at  once.  Now,  this  equal  balance  is  found 
in  the  skeleton,  and  especially  in  the  limbs.  In  the  »ius- 
cular  system  the  adaptive  modification  is  too  extreme  ;  it 
obscures  the  derivative  origin.  In  the  case  of  the  vis- 
ceral system^  on  the  contrary,  there  is  scarcely  enough 
modification  to  make  any  evidence.  Next  to  the  skele- 
tal, the  best  evidences  are  found  in  the  nervous  system. 
Here  the  essential  identity  of  parts  in  all  vertebrates, 
and  yet  their  modification  in  each  class,  is  very  striking, 
especially  in  the  brain.  We  have  already  explained  this 
(pages  76-78).  In  the  case  of  the  muscular  system  the 
modification  in  passing  from  the  fish  to  the  land  verte- 
brates is  so  extreme  that  all  hope  of  homology  seems 
vain.  But  in  land  vertebrates,  from  the  amphibia  (frogs, 
etc.)  to  man,  it  may  probably  be  traced  by  careful  study, 
but  this  has  not  been  attempted,  except  in  a  fragmentary 
way.     Undoubtedly  a  rich  field  is  open  here. 

in.     SERIAL    HOMOLOGY. 

There  is  an  evident  correspondence  in  the  several 
parts  of  the  fore  and  hind  limbs.  The  hip  girdle  corre- 
sponds with  the  shoulder  girdle,  and  bone  for  bone, 
although  the  parts  are  more  consolidated  in  the  former; 
the  femur  corresponds  to  the  humerus  ;  the  two  bones 
of  the  leg  to  the  two  bones  of  the  forearm,  each  to  each  ; 
the  seven  bones  of  the  ankle  to  the  eight  bones  of  the 
wrist,  two  of  the  former  having  been  consolidated  into 
one;  the  five  bones  of  the  instep  (metatarsal)  to  the  five 
bones  of  the  palm  (metacarpal)  and  the  fourteen  bones 
of  the  toes  to  the  fourteen  bones  of  the  fingers.  This 
introduces  us  to  the  idea  of   a  serial  repetition  of  similar 


262    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


v.b 


parts — i.  e.,  of  serial  homology.  In  general  terms  as  ap- 
plied to  the  whole  skeleton,  it  may  be  stated  thus:  As 
the  whole  vertebrate  kingdom  is  made  up  of  a  repeti- 
tion of  individuals  constructed  on  the  same  plan,  but 
modified  according  to  the  place  and  function  in  the  verte- 
brate scale,  so  each  individual  vertebrate  is  made  up  of  a 
repetition  of  segments,  similar  in  plan  but  modified  ac- 
cording to  its  place  and  function  in  the  series  of  segments. 
This  is,  of  course,  best  brought  out  in  the  skeleton. 

Take  any  vertebrate,  such  as  a  fish  or  a  man.  Make 
a  cross  section  of  the  body  and  look  at  the  end.  What 
we  see  is  shown  diagrammatically  in  Fig.  i66.  The  sec- 
tion of  the  skeleton  consists 
of  three  parts — centrum,  c,  an 
arch  above  (neural  arch,  n^ 
surrounding  nervous  centers, 
and  an  arch  below  (visceral 
arch,  v)  surrounding  the  vis- 
ceral cavity.  The  whole  is 
enveloped  in  muscle  and  skin, 
with  which  we  have  no  con- 
cern now.  The  three  parts 
named  above  constitute  one 
segment  of  the  skeletal  axis, 
and  may  be  called  a  vertebra. 
Now  the  whole  skeletal  axis 
may  be  regarded  as  made  up 
of  a  repetition  of  such  skeletal 
segments  or  vertebra  modi- 
fied according  to  place  and  function  in  the  series  (Fig. 
167).  The  centrums  repeated  form  the  vertebral  col- 
umn, the  neural  arches  repeated  constitute  the  neural 
canal,  in  which  are  lodged  the  nervous  centers,  and  the 
visceral  arches  repeated  make  the  visceral  cavity.  But 
these  segments  are  modified  according  to  the  place  in 


Fig.  166. — Cross  section  throug-h 
a  fish  :  v^  visceral  system ; 
c,  vertebra  ;  b.  blood  system  ; 
«,  nervous  system. 


GENERAL    LAWS   OF   ANIMAL   STRUCTURE.      263 

the  series,  sometimes  by  enlargement,  sometimes  by 
diminution  and  even  disappearance,  sometimes  by  con- 
solidation  of  several  together.     But  throughout  these 


Fig.  167. — Owen's  archetypal  vertebrate,  showing  the  successive  segments 
but  slightly  modified.  The  olfactory  capsule,  the  eye,  and  the  ear  are 
represented  detached. 

modifications  the  essential  identity,  although  obscured, 
is  still  discernible. 

For  example:  In  the  dorsal  region  of  the  higher 
vertebrates  the  visceral  arches  are  greatly  enlarged  to 
contain  the  thoracic  viscera.  Going  forward,  in  the  neck 
region  in  man  and  most  vertebrates  the  visceral  arches 
are  wanting  in  order  to  give  greater  freedom  of  motion, 
but  not  in  a//  vertebrates,  nor  in  all  stages  of  develop- 
ment. Ribs  are  borne  by  all  the  vertebrse  up  to  the 
head  in  fishes,  in  serpents,  and  also  in  the  embryos  of 
mammals,  and  even  of  man.  Going  still  forward,  accord- 
ing to  one  view,  the  head  is  composed  of  several  vertebra's 
consolidated  and  greatly  modified  (see  Fig.  167).  Ac- 
cording to  this  view,  the  basilar  portions  of  the  occipital 
and  sphenoid,  and  the  ethmoid  are  the  centrums  of  three 
successive  vertebras,  the  neural  arches  of  which,  greatly 
enlarged,  form  the  skull,  while  the  diminished  and  much 
modified  visceral  arches  make  up  the  face.  There  is, 
however,  some  doubt  whether  the  vertebral  theory  of 
the  skull  is  true  in  this  strict  sense;  and,  if  so,  of  how 
many  vertebrae  it  consists. 

Going  now  backward,  in  the  lumbar  region,  again,  the 
ribs  are  wanting  in  man  and  mammals,  but  not  in  fishes, 


264    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 

nor  in  serpents,  nor  in  tailed  amphibians,  nor  in  the  em- 
bryos of  mammals,  nor  even  in  the  embryo  of  man.  So 
that  their  absence  is  only  an  extreme  term  of  modifica- 
tion, all  the  steps  of  which  may  be  found.  Going  still 
back,  the  sacrum  may  be  regarded  as  five  to  six  vertebrae 
with  their  bodies — neural  arches  and  visceral  arches — all 
consolidated.  Indeed  they  are  distinct,  and  have  their 
ribs  in  many  lower  vertebrates  and  in  the  embryos  of 
mammals  and  man.* 

Signification  of  Limbs. — It  will  be  observed  that 
in  this  scheme  we  have  left  out  the  limbs.  This  seems 
a  very  great  omission,  but  this  is  so  only  in  the  higher 
vertebrates.  The  primal  vertebrates  were  probably  limb- 
less. And  in  all  the  earliest  vertebrates  and  in  the 
lowest  vertebrates  to-day  limbs  are  comparatively  in- 
significant appendages,  which  may  be  left  out  in  any 
general  scheme  of  skeletal  structure.  It  is  probable 
that  limbs  ca/i  not  be  brought  into  the  original  plan  of 
homologous  segments,  but  have  been  added  afterward 
as  a  sort  of  afterthought. 

Origin  of  Limbs. — Professor  Owen  f  thought  to 
bring  limbs  into  this  scheme  of  repeated  segments  by 
making  them  appendages  to  the  visceral  arches,  and 
this  view  is  expressed  in  his  figure  of  the  archetypal  ver- 
tebrate (Fig.  167),  where  small  appendages  are  seen  on 
every  arch.  He  believed  that  the  shoulder  girdle  and 
hip  girdle  were  formed  by  consolidation  of  several  vis- 
ceral arches,  and  that  the  limbs  were  greatly  enlarged 
appendages  to  these  arches,  the  appendages  to  the 
other  arches  being  still  rudimentary  or  wanting.  But 
this  view,  though  very  suggestive,  is  not  now  generally 

*  Cervical,  lumbar,  and  sacral  ribs  are  found  in  the  embryo  of 
mammals  and  of  man.     Wiedersheim,  p.  51. 

f  Owen,  "  Homologies  of  Vertebrate  Skeletons  "  and  "  Signifi- 
cation of  Limbs." 


GENERAL   LAWS   OF  ANL\L\L    STRUCTURE.      265 

accepted.  The  more  probable  view  is  that  the  same  fold 
of  the  skin  which  formed  the  lower  unpaired  fin  (anal 
fin)  divided  as  it  came  forward  into  two  lateral  folds 
(Fig.  168,  A),  that  these  by  local  enlargements  became 
the  paired  fins,  anterior  and  posterior  (Fig.  168,  B),  and 
by  addition  of  skeletal  framework  were  developed  into 


Fig.  168. — Diagrams  showing  probable  origin  of  limbs  :  A,  earliest  stage, 
showing  lateral  folds  ;  B,  later  stage,  after  paired  fins  were  formed  (from 
Wiedersheiraj  ;  C,  the  formation  of  limbs  from  fins  (from  Parker). 


limbs  (Fig.  168,  C)  and  finally  became  connected  with 
the  skeletal  axis  for  more  effective  action.  The  subject 
is,  however,  still  obscure. 

Serial  Homology  of  Other  Systems. — Other  sys- 
tems show  segmented  structure,  but  not  so  clearly. 
Next  to  the  skeletal  in  this  regard  comes  the  nervous 
system.  Thus  we  may  conceive  the  cerebro-spinal  axis 
as  a  series  of  ganglia  (fused,  however,  into  a  continuous 
tract)  corresponding  to  the  vertebrae,  and  giving  off  each 
a  pair  of  nerves;  and,  according  to  the  vertebral  theory 


266    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

of  the  head,  the  successive  lobes  of  the  brain  (cerebel- 
lum, optic  lobes,  cerebrum,  and  olfactory  lobes)  corre- 
spond to  the  successive  cranial  vertebrae. 

The  muscular  system  shows  segmentation  in  a  marked 
degree  in  fishes  and  some  lizards.  The  flakes  of  a  fish's 
flesh  correspond  to  the  vertebrae,  and  we  have  here  lit- 
erally body  segments  {somites).  See  also  the  so-called 
glass  snake — really  a  limbless  lizard.  Under  a  sharp 
blow  the  tail  breaks  into  segments  right  through  be- 
tween scales,  flesh  flakes,  and  vertebrae. 

In  the  visceral  system  the  segmentation  is  not  dis- 
cernible. 

As  a  general  law,  the  number  of  segments  and  their 
similarity  is  greater  as  we  go  down  the  scale  of  verte- 
brates, and,  contrarily,  as  we  go  up  the  scale  adaptive 
modification  obscures  more  and  more  the  homology. 

SECTION    III. 

Invertebrates. 

I.    ARTICULATA. 

Under  Articulata  we  include  arthropods  and  annelids 
or  worms  (see  schedule,  page  7 1),  because,  from  the  homo- 
logical  point  of  view  (though  not  from  any  other),  they 
are  best  united.  What  we  propose  to  prove  is  that  the 
structure  of  all  these  animals  is  exactly  such  as  it  would 
be  if  they  all  came  by  descent,  with  modifications,  from 
one  primal  articulate  animal;  and,  therefore,  that  we 
must  conclude  that  they  did  all  so  come. 

General  Plan. — The  general  plan  of  structure  is  the 
same  throughout,  but  wholly  different  from  that  of  ver- 
tebrates. I.  The  skeleton  is  external,  instead  of  internal. 
2.  The  nervous  system  lies  along  the  ventral,  instead  of 
the  dorsal,  aspect  of  the  body.  3.  The  body  consists  of 
one  cavity,  instead  of  two. 


GENERAL   LAWS   OF   ANIMAL   STRUCTURE.      267 

Of  these  two  positions  of  the  skeleton,  which  is  best  ? 
For  motion  and  locomotion  they  are  probably  equally 
good  ;  but  the  external  skeleton  has  the  great  advantage 
of  being  protective  as  well  as  locomotive,  while  the  in- 
ternal skeleton  has  the  much  greater  advantage  of  leav- 
ing the  surface  sensitive  to  external  impressions,  and 
therefore  an  inlet  to  knowledge  of  external  nature, 
and  thus  gives  greater  capacity  for  higher  development. 

So  much  for  the  general  plan.  Now  the  details  of 
the  homology. 

Serial  Homology. — In  the  case  of  vertebrates  we 
took  up  first  special  homology,  because  this  is  by  far  the 


Fig.  169. — Diag:ram  section  across  an  arthropod,  showing  the  inclosing 
skeleton  ring  and  a  pair  of  jointed  appendages :  //,  nervous  center ; 
V,  viscera  ;  d,  blood  system. 


most  distinct.  In  the  case  of  Articidata  we  take  up  only 
serial  homology,  because  not  only  is  this  the  most  dis- 
tinct, but  it  includes  the  other  also;  for  we  may  regard 
each  articulate  animal  as  consisting  of  a  series  of  seg- 
ments essentially  similar,  but  modified  according  to  its 
place  and  function  in  the  series,  and  then  the  structure 
thus  formed  is  again  modified  according  to  the  place  in 


268   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

the  scale  of  Articulata,  to  make  the  infinite  variety  of 
forms  constituting  this  department  of  animals. 

Our  object,  then,  is  to  show  that  the  skeleton  of  an 
arthropod  consists  of  naught  else  than  a  series  of  similar 
segments,  modified  according  to  the  place  and  function 
in  the  series. 

Take  any  articulate  animal,  say  an  arthropod,  like  an 
insect  or  crustacean.  Make  a  cross  section  and  view  it  on 
end  (Fig.  169).  We  see  an  external  bony  ring  inclosing 
everything,  and  attached  to  it  a  pair  of  jointed  append- 
ages, right  and  left.  Now,  the  whole  skeleton  is  made  up 
of  such  rings  and  appendages,  repeated  and  modified.  The 
repetition  of  the  rings  gives  rise  to  a  hollow,  many-jointed 
cylinder  or  barrel,  and  the  repetition  of  the  appendages 
to  a  continuous  row  of  such  on  each  side.  Each  ring, 
with  its  pair  of  appendages,  is  called  a  somite,  or  body 
segment.  Such  is  the  simple  idea  or  archetype  of  an  ar- 
ticulate animal ;  but,  in  fact,  these  ideal  rings  and  append- 
ages are  very  variously  modified  for  function.  Some 
are  modified  for  swimming  appendages,  some  for  walk- 
ing appendages  or  limbs,  some  as  food-gathering  append- 
ages (jawfeet),  some  as  biting  appendages  (jaws),  some 
as  sense  appendages  (eyes,  ears,  feelers),  but  all  made 
on  the  same  plan,  only  modified.  The  mianner  of  modifi- 
cation is  sometimes  by  enlargement,  sometimes  by  dimi- 
nution and  even  disappearance,  sometimes  by  coalescence 
and  consolidation  of  several  into  one  piece. 

For  example,  take  one  from  about  the  middle  of  the 
scale — say,  a  crawfish  or  a  lobster.  This  animal  consists 
of  about  twenty-one  rings  and  pairs  of  appendages  (Fig. 
170).  In  the  tail  region  or  abdomen  the  seven  rings 
are  all  separate  and  but  little  modified,  but  their  jointed 
appendages  are  greatly  modified  for  swimming,  D' ; 
those  of  the  last  joint  but  one  are  enlarged  and  flat- 
tened, and,  together  with  the  flattened  last  joint,  which 


Fig. 


1 70. -External  anatomy  of  the  lobster,     r After  Kingsley.^ 


270  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


is  without  appendages,  form  the  powerful  flipper  at  the 
end  of  the  tail  for  backward  darting.  Going  thence 
forward,  we  come  to  the  cephalo- 
thorax.  Here  many  joints  in  their 
dorsal  parts  are  consolidated  into 
V  a  carapace,  and  their  ringed  struc- 
.1  ture  is  lost,  but  their  entire  dis- 
^  tinctness  is  evident  in  the  lower 
crustaceans  and  in  the  embryos 
of  the  higher.  But  observe  that 
it  is  only  the  dorsal  parts  that  are 
^   consolidated.    In  the  ventral  parts 

1  the  separate  rings  are  distinct, 
^  and  each  has  its  own  pair  of 
^    jointed    appendages    greatly    en- 

I  -  larged  for  walking,pj\f\f\f\ 
%  These  are  the  five  pairs  of  limbs. 
a.  Going  still  forward,  we  find  next 
^  three  or  four  pairs  modified  for 
^  the  gathering  of  food.  These 
rt  are  called  maxillipeds^  or  jawfeet, 
£  md.  ind"^,  nuP .  The  modification  is 
^  <  not  so  great  but  that  their  resem- 
1^  y  blance  to  the  limbs  is  obvious. 
"    Next  come  two  or  three  pairs  more 

2  modified,  so  as  to  adapt  them  for 
biting.  These  are  the  maxil/a;,  or 
jaws,  mx.  Next  come  two  pairs 
of  highly  modified,  greatly  elon- 

^  gated,  and  many-jointed  append- 
ages, which  are  organs  of  touch 
and  hearing.  They  are  sense  ap- 
pendages.    Lastly,  a  pair  of  joint- 

j;  ed  appendages,  on  the  ends  of 
Qp       J  c^        which  are  placed  the  eyes.     Some 


GENERAL  LAWS  OF  ANLMAL  STRUCTURE. 


271 


doubt  whether  these  belong  to  the  same  category  as  the 
other  appendages,  but  they  are  usually  so  regarded.  In 
Fig.  171  we  give  the  whole  series  of  appendages  in  the 
crawfish  : 

The  crab  is  much  more  modiiied,  and  the  consolida- 
tion more  complete.  The  tail  seems  to  be  absent,  but 
is  really  only  diminished  in  size  and  bent  under  and  con- 
cealed beneath  the  body ;  but  in  the  embryo  the  tail  is 
similar  to  that  of  the  crawfish  (Fig.  172).     The  maxilli- 


FiG.  172. — Development  of  Carcinus  moenas  :  A,  zoa>a  stage  ;  B,  megalopa 
stage  ;  C,  final  state.     (After  Couch.) 


peds  and  sense  appendages  are  also  much  reduced  in  size, 
but  they  are  all  present. 

Going  down  the  Scale. — We   have   taken  a  case 

from  about  the  middle  of  the  articulate  scale,  because 
there  the  essential  identity  and  adaptive  modification 
are  evenly  balanced  and  both  conspicuous.  But  the  evi- 
dence is  completed  by  going  down  and  up  the  scale.  In 
the  lower  crustaceans  the  rings  are  all  separate  and  the 
appendages  less  and  less  modified  (Fig.  173).  A  series 
19 


2^2    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

of  appendages  of  a  still  lower  form  is  given  in  Fig.  174. 
In  this  case  the  appendages  are  too  much  alike  to  be 
easily  classified.  Often,  too — e.  g.,  in  the  Liniulus — the 
appendages  which  in  the  higher  crustaceans  are  walking 
appendages  become  swimmers,  while  the  maxillipeds  be- 
come walkers.  Finally,  in  the  lowest  crustaceans  and, 
still  better,  in  the  centipede  and  in  some  marine  worms, 
the  rings  and  appendages  are  greatly  increased  in  num- 
ber, and  are  so   similar  that  there   is  not  modification 


Fig.  173. — Vibilia,  an  amphibod  crustacean.     (After  Milne  Edwards.) 

enough  to  furnish  argument  for  homology.  There  is 
only  a  slight  modification  of  the  head  and  tail  joints 
(see  Fig.  175).  Indeed,  some  marine  worms  multiply 
by  self-division.  In  such  cases  at  the  point  of  division 
some  of  the  rings  are  consolidated  and  appendages 
modified  to  form  a  new  /lead  and  a  new  tail  (Fig.  176), 

Going  up  the  Scale. — Going  up  the  scale  to  in- 
sects the  modification  is  greater,  but  the  elements — viz., 
rings  and  appendages — are  the  same.  An  insect  consists 
ideally  of  about  sixteen  or  seventeen  segments  and  ap- 


GENERAL   LAWS   OF  ANLNLXL   STRUCTURE. 


-/i 


--< 


pendages  (Fig.  177).  In  the  abdomen  the  rings  are 
perfect  and  movable,  although  the  appendages  are 
wanting;   but  they  are  present  in   the  -^ 

larval  or  caterpillar  state.     The  thorax  '^^ 

is  a  consolidation  of  three  rings,  each  ^^^ 

with  its  pair  of  appendages  greatly  en- 
larged for  walking  (legs).  The  head 
consists  of  three  or  four  consolidated 
segments  with  appendages  much  modi- 
fied— the  first  pair  into  antennae,  the 
second  pair  into  mandibles,  and  the 
third  pair  into  maxillae  and  maxillary 
appendages  or  feelers,  and  the  fourth 
pair  into  labium  and  labial  append- 
ages. 

Origin  of  Insects'  Wings.— The 
wings  of  insects  are  not  homologous 
with  legs  or  other  appendages.  There 
is  some  doubt  as  to  their  origin,  but  it 
is  most  probable  that  they  are  modifi- 
cations of  the  trachi-branchise  of  the 
larvae  of  aquatic  forms.  If  so,  insects 
sprang  from  aquatic  species. 

That  insects  are  higher  than  crusta- 
ceans is  shown  by  the  distinctness  of  the 
head.  The  ring-series  in  crustaceans 
are  grouped  into  two  regions,  the  ab- 
domen and  the  cephalothorax,  the  head 
being  undistinguishable  from  the  tho- 
rax. The  same  is  true  of  spiders  and 
scorpions.  But  in  insects  we  have  three 
distinct  groups — the  head,  the  thorax, 
and  the  abdomen.  A  distinct  movable 
head  is  always  a  sign  of  the  dominance 
of  head  functions. 


£^ 


274 


PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


The  higher  position  of  insects  is  further  shown  by  the 
number  of  their  legs  or  appendages  used  for  locomotion. 
It  is  a  law  in  biology  that  a  great  number  of  parts,  simi- 


FiG.   175. — Lithobius  forcipatus.     (After  Carpenter.) 

lar  in  form  and  function  (vegetative  repetition),  indicates 
a  low  position  in  the  scale  of  organisms.  As  we  go  up 
the  scale  the  number  of  parts  used  for  one  function  be- 
comes less,  and  their  efficiency  becomes  correspondingly 
greater.  Legs  are  an  admirable  illustration  of  this  law. 
In  marine  worms  and  in  the  lowest  Crustacea  there  is  an 
indefinite  but  very  great  number  of  similar  legs.  As  we 
rise  among  Crustacea  the  number  becomes  definite,  and 
countable  as  legs,  when  there  are  fourteen  or  seven  pairs. 
These  are  called  tetradecapods.  In  the  higher  Crusta- 
cea— crabs,  crawfish,  etc. —  they  are  reduced  to  five  pairs. 


Fig.   176. — Syllis  prolifera. 


These  are  therefore  called  decapods.  In  spiders  and  scor- 
pions there  are  only  four  pairs.  These  might  be  called 
octopod  insects.     In  true  insects  they  are  reduced  to  three 


GENERAL   LAWS   OF  ANIMAL   STRUCTURE.     275 


pairs,  and  these  are  called  hexapods.     If  it  be  allowable 
to  pass  from  one  department  to  another,  we  might  go 


(XtvteiiUV;^ 


TvbvaxX-  p^^^^  tm^m.  <P% 


/ 


<=fr=-sa 


•^ab^Lovnew, 


eutvic    JceitttiO'^S, 


f     Q 


SteTKVvU     ^      lo-^^^-P-  K 


Fig.  177. — External  anatomy  of  Caloptenus  spretus,  the  head  and  thorax 
disjointed  :  ///,  uropatagium  ;  y,  furcula ;  c,  cercus.  (Drawn  by  J.  T. 
Kingsley. ) 


2-6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

on  and  say  that  in  vertebrates  the  limbs  are  reduced  to 
two  pairs,  and  these  are  therefore  quadrupeds.  This  is  as 
far  as  reduction  can  go  for  highest  locomotive  efficiency 
on  land.  In  birds  locomotive  efficiency  on  land  is  sacri- 
ficed for  flight;  and  in  man  it  is  sacrificed  for  efficiency 
of  another  and  still  higher  kind,  and  the  reduction  goes 
on  to  one  pair,  the  other  pair  common  to  vertebrates 
being  set  free  for  higher  uses  as  wings  and  hands. 

Law  of  Differentiation. — We  have  already  seen 
(page  23)  that  cells,  commencing  in  the  lowest  animals 
and  in  the  earliest  embryonic  condition,  all  alike,  and  per- 
forming each  all  the  functions  of  life  but  very  imper- 
fectly, as  we  rise  in  the  scale  become  differentiated  in 
form  and  specialized  in  function,  each  doing  its  own 
work  and  doing  it  much  better.  So  now  we  find  the 
same  law  in  the  segments.  In  the  earliest  geological 
times,  and  in  the  lowest  animals  now,  we  find  all  the 
segments  alike  and  performing  similar  functions,  but 
imperfectly.  But  as  we  go  up  the  scale  the  segments 
and  their  appendages  are  more  and  more  differentiated 
in  form  and  function,  and  by  this  division  of  labor  the 
functions  are  better  performed.  The  same  law  may  be 
carried  still  a  step  further.  Regarding  a  whole  depart- 
ment, such  as  the  vertebrata  or  articulata,  as  composed 
of  a  repetition  of  organisms  made  on  the  same  plan  ; 
then  in  early  geological  times  there  was  doubtless  far 
more  similarity  than  now.  In  the  process  of  evolution 
these  somewhat  similar  organisms  were  differentiated, 
each  kind  for  its  own  place  in  the  economy  of  Nature, 
and  specially  fitted  for  that  place  through  the  survival 
of  the  fittest.  This  law  of  differentiation,  therefore, 
is  the  most  fundamental  and  all-pervasive  law  of  evo- 
lution. 

Nervous  System. — We  have  confined  ourselves  thus 
far  to  the   skeletal  system.     Next  to  this,  as  illustrat- 


GENERAL    LAWS   OF  ANIMAL   STRUCTURE. 


277 


ing  serial  repetition,  would  come  the  nervous  system. 
Ideally  the  nervous  system  of  articulata  consists  of  a 
series  of  ganglia,  one  to  each  somite  or  body  segment 
and  presiding  over  that  segment,  but  sympathetically 
connected  by  a  continuous  thread  with  one  another,  and 
all  with  the  cephalic  ganglion.  But  as  we  pass  up  the 
scale,  modification  and  consolidation,  differentiation  and 
specialization,  proceed  pari  passu,  as  already  shown  on 
page  87. 

2.    ISIOLLUSCA. 

Here,  again,  we  find  an  entirely  different  general  plan 
of  structure.  In  vertebrates  we  have  an  internal  skele- 
ton, the  axis  of  which  consists  of  segments  modified  ac- 
cording to  place  and  function.  In  articulates  we  have 
an  external  skeleton  still  more  obviously  segmented  and 
modified.  In  moUusca  alone,  among  all  the  great  de- 
partments, we  have  no  segmentation,  no  serial  repetition 
of  ideally  similar  parts.  This  want  of  repeated  seg- 
ments is  its  most  distinctive  peculiarity.  In  mollusca, 
also,  we  have  no  true — i.  e.,  locomotive — skeleton  at  all, 
either  internal  or  external,  but  only  a  protective  shell. 

General  Character. — The  general  characters  of  this 
type  are  mostly  negative:  i.  A  want  of  a  true  locomo- 
tive skeleton.  2.  A  complete  want  of  segmentation  of 
any  system.  3.  The  presence  of  a  soft,  mucous,  sensi- 
tive surface  wherever  not  covered  with  shell. 

The  most  important  of  these  is  the  complete  absence 
of  segmentation.  It  follows  from  this  that  there  can  be 
no  serial  homology.  There  is,  of  course,  a  special  ho- 
mology— i.  e.,  a  homology  of  part  with  part  in  different 
orders  and  classes  of  mollusks,  but  it  is  obscure  and 
little  studied.     I  shall  not  attempt  its  exposition. 

]]'hy  there  is  no  Segmentation  in  this  Type. — It  is  seen 
that  homology,  especially  serial  homology,  is  mostly  lim- 
ited to  the  skeleton  and  nervous  system — i.  e.,  to  the  or- 


2/8 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


gans  concerned  with  the  distinctive  anivial  life.  Animal 
Ufe  and  activity  has  to  do  specially  with  action  and  reac- 
tion between  the  animal  and  its  environment.  The  limbs 
especially  take  hold  on  the  environment  and  are  modi- 
fied by  it.  The  vegetative  organs,  on  the  contrary,  are 
little  affected  by  the  environment.  Segmented  structure 
is  peculiarly  adapted  for  diversified  modification,  showing 
homology.  Now,  in  articulates  the  distinctive  animal 
functions  are  very  highly  developed  in  proportion  to  the 
vegetative,  and  these  are  therefore  the  most  perfectly 
segmented  of  all  animals.  In  mollusks,  on  the  contrary, 
the  vegetative  functions  are  very  highly  developed  in 
proportion  to  the  distinctively  animal,  and  these  do  not 
lend  themselves  readily  to  homology.  The  vertebrates 
seem  in  this  regard  to  combine  the  characters  of  both. 

3.    RADIATA. 

In  these,  including  echinoderms  and  coelenterates, 
we  again  find  an  entirely  different  type  of  structure.  We 
find,  indeed,  segmentation  again,  but  of  an  entirely  dif- 
ferent kind.  All  these  animals  have  an  essentially  radi- 
ated structure.  Like  the  plugs  of  an  orange  arranged 
about  the  central  pith,  or  like  the  spokes  and  fellies  of 
a  wheel  about  the  central  hub,  so  are  the  several  seg- 
ments of  a  radiated  animal  symmetrically  arranged  about 
its  central  mouth  and  stomach,  and  all  the  organs  of  the 
body  are  repeated  in  each  segment.  Thus,  in  a  starfish, 
for  example,  the  mouth  and  the  stomach  are  in  the  cen- 
ter and  surrounded  by  the  nerve  centers  (oesophageal 
collar)  and  by  the  blood  centers.  From  this  center  go 
to  each  arm  or  segment  a  branch  of  the  stomach,  a 
branch  of  the  nervous  system  from  a  corresponding  gan- 
glion of  the  collar,  and  a  great  branch  of  the  blood  sys- 
tem. Each  arm  contains  also  its  own  equal  i^2iX\.  of  the 
respiratory   system  and  the   reproductive  system.     The 


GENERAL  LAWS  OF  ANIMAL  STRUCTURE. 


279 


radial  arrangement  of  the  nervous  system  is  shown  in 
Fig  67,  page  92.  The  other  systems  will  be  illustrated 
hereafter. 

Comparison  of  this  with  Other  Types. — Com- 
paring this  with  other  types,  in  both  vertebrates  and 
articulates  we  have  segments  repeated  in  a  linear  series, 
and  therefore  an  anterior  d^nd  posterior  extremity;  but  in 
this  we  have  segments  repeated  in  a  circular  series,  and 
therefore  no  beginning  and  no  end,  no  anterior  and  no 
posterior  extremity.  Again,  in  all  other  types,  includ- 
ing mollusca,  we  have  all  the  organs,  especially  those  of 
animal  life,  repeated  on  the  two  sides  of  a  median  plane 
— i.  e.,  bilateral  symmetry ;  in  this  one  we  have  repetition 
of  organs  about  a  central  column — i.  e.,  radial  symme- 
try. In  the  highest  radiates  alone  are  found  the  distinct 
beginnings  of  a  bilateral  symmetry. 

In  this  also,  as  in  the  other  two  segmented  types,  the 
completeness  of  the  segmentation,  the  number  of  re- 
peated parts,  and  their  similarity  are  greatest  in  the 
lower  part  of  the  scale ;  and  as  we  rise  the  segments 
become  more  and  more  dissimilar  by  modification  for 
various  functions.  But  the  obscuration  by  modification 
is  less  conspicuous  in  this  type,  because,  taken  as  a  whole, 
the  animals  of  this  are  less  highly  organized. 

4.    PROTOZOA. 

Among  these,  of  course,  there  is  as  yet  no  distinct 
plan  of  structure  (for  they  consist  of  a  single  cell),  and 
structure  such  as  we  have  been  speaking  of  is  produced 
by  differentiation  of  the  elements  of  a  cell  aggregate. 
These  lowest  animals  may  be  regarded  as  the  living 
stuff  out  of  which  the  different  types  were  constructed — 
the  trunk  from  which  diverged  the  four  great  branches 
(F'ig.  158,  page  248).  There  is  no  room  for  homology 
here. 


28o  PHYSIOLOGY   AND    MORPHOLOGY    OF   ANLMALS. 


GENERAL    CONCLUSIONS. 

The  general  conclusions  of  this  chapter  may  be 
briefly  summarized  in  several  propositions. 

I.  There  are  at  least  four  very  distinct  plans  of 
structure  among  animals  through  which  homology  may 
be  more  or  less  clearly  traced,  but  beyond  which  it  can 
not  be  distinctly  traced,  although  it  doubtless  exists. 
These  are  the  vertebrata,  articulata,  mollusca,  and  radi- 
ata.  The  characteristic  plan  of  vertebrates  is  that  of 
an  internal  skeleton,  the  axis  of  which  consists  of  seg- 
ments ideally  similar,  but  modified  according  to  the  place 
and  function  in  the  series  of  segments.  The  animal 
thus  formed  is  again  modified  according  to  its  place  in 
the  scale  of  vertebrates. 

The  characteristic  plan  of  the  articulata  is  that  of  an 
external  skeleton,  composed  of  segments  and  pairs  of 
appendages  ideally  similar,  but  modified  according  to 
the  place  in  the  series.  The  annnal  thus  formed  is 
again  modified  according  to  its  place  in  the  scale  of 
articulata. 

The  characteristic  plan  of  the  mollusca  is  the  total 
want  of  a  true  locomotive  skeleton,  and  especially  the 
entire  absence  of  segmentation.  There  is  therefore  no 
room  for  serial  homology.  Even  special  homology  is 
more  obscure  in  this  than  in  other  departments.  Nev- 
ertheless, there  is  enough  to  assure  us  that  these  also 
came  by  modification  from  some  primal  form  of  mol- 
lusk. 

The  characteristic  plan  of  radiata  is  that  of  similar 
segments  arranged  symmetrically  about  a  central  mouth 
and  stomach.  In  vertebrates  and  articulates  we  have 
segments  repeatecf  in  a  linear  series  ;  in  radiates  in  a  cir- 
cular series.  In  all  other  types  we  have  dilateral  symme- 
try ;  in  this  we  have  radial  symmetry. 


GENERAL    LAWS    OF   AMMAL    STRUCTURE.      28 1 

2.  In  all  plans  the  ideal  similarity  of  the  repeated 
parts  in  the  animal  and  of  the  repeated  animals  in  the 
department  is  clearest  in  the  lower  part  of  the  scale,  and 
adaptive  modification  for  function  becomes  more  con- 
spicuous as  we  rise,  until  finally  the  essential  identity 
may  be  wholly  obscured  by  adaptive  modification.  This 
is  the  law  of  progressive  differentiation  so  universal  in 
Nature. 

3.  All  these  phenomena  may  be  completely  explained 
by  the  origin  of  organic  forms  by  derivation — i.  e.,  by 
"  descent  with  modifications  "  ;  in  a  word,  by  the  theory 
of  evolution,  and  can  not  be  e.xplained  in  any  other  way. 


PART    II. 

ORGANS  AND  FUNCTIONS  OF  ORGANIC  LIFE. 

We  have  explained  (page  24)  that  all  the  functions 
of  the  animal  body  fall  into  two  groups — one,  distinctive 
of  animals,  and  therefore  called  the  functions  of  animal 
life;  the  other,  possessed  in  common  with  plants,  and 
called  functions  of  vegetative  or  organic  life.  We  have 
now  treated  of  the  distinctively  animal  functions.  We 
come  now  to  treat  of  the  functions  of  organic  life  and 
their  organs. 

These  are  again  subdivided  into  two  very  distinct 
groups — viz.,  the  nutritive  and  the  reproductive — the 
one  including  all  that  assemblage  of  functions  which  con- 
tribute to  the  conservation  of  the  individual  life;  the 
other,  all  that  assemblage  of  functions  which  insures  the 
continuance  of  the  species.  We  reproduce  here,  with 
slight  addition,  the  schedule  already  used  on  page  24: 

f  .  (  Sensation  and  consciousness. 

Ani-     Functions  of  animal  life..    -  ^^  v..-  j       1      ^ 

I    I  {  Volition  and  voluntary  motion. 

body   i  \  Nutritive  -^  Nutrition  proper. 

[  Functions  of  vegetative  life  <  {  Elimination. 

(  Reproductive 

2S2 


CHAPTER    I. 

NUTRITIVE     FUNCTIONS — METABOLISM,    OR    WASTE    AND 
SUPPLY. 

Coextensive  with  life  and  lying  at  the  very  basis 
of  all  life  phenomena  is  a  continuous  change  by  waste 
and  supply  of  the  material  of  which  the  body  is  com- 
posed. This  whole  process  of  change  is  called  metabo- 
lism, ox  transformation.  It  is  not  only  coextensive  with 
life,  but  it  may  be  said  to  be  life  itself.  It  consists 
necessarily  of  two  parts — viz.,  an  ascensive  change,  by 
which  new  tissue  is  formed  from  crude  material,  and  a 
descensive  change,  by  which  old  tissue  is  decomposed 
and  eliminated  from  the  body.  The  former  is  called 
anabolisni,  the  latter  katabolism,  or  waste.  This  latter  is 
apparently  the  active,  initiative  agent  of  the  whole  pro- 
cess. 

"Waste. — The  process  of  waste  is  so  fundamental 
that  it  must  be  thoroughly  illustrated. 

I.  Suppose,  then,  we  had  a  pair  of  scales  of  enormous 
size,  and  one  of  you  (hearers  or  readers)  were  lying  in  a 
comfortable  position  in  one  pan  and  a  weight  for  perfect 
counterpoise  in  the  other.  I  shall  suppose  you  at  per- 
fect rest  physically  and  peace  mentally,  and,  as  contrib- 
uting to  this  condition,  perhaps  smoking  a  cigar.  The 
equilibrium  would  not  continue  indefinitely;  if  the  scales 
were  delicate,  not  even  for  a  minute.  On  the  contrary, 
even  while  we  watch  the  experiment,  your  side  of  the 

283 


284   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

scale  goes  up.  The  body  is  consuviing  like  the  cigar — 
literally  burning  up  and  passing  away  as  invisible  gas 
through  the  nostrils.  Probably  about  two  pounds  is 
thus  consumed  in  twenty-four  hours.  This  process  of 
waste  is  more  rapid  in  higher  than  in  lower  animals.  It 
is  continuous  with  life  and  in  proportion  to  its  grade. 
This  may  be  expressed  by  the  formula: 

I.   Waste  cc  Life. 

2.  We  have  taken  the  case  of  absolute  rest  of  body 
and  mind.  But  suppose  next  that  the  subject  of  this 
experiment  is  in  violent  activity,  physical,  mental,  or 
moral,  perhaps  dancing  a  hornpipe,  perhaps  wrestling 
with  a  mathematical  problem,  perhaps  in  intense  anx- 
iety, remorse,  or  grief.  Under  these  conditions  the  wast- 
ing, as  measured  by  pounds,  is  more  rapid.  This  is  ex- 
pressed by  formula  2  : 

2.   Waste  oc    Work. 

3.  We  will  next  suppose  the  subject  to  be  inclosed  in 
a  large  calorimeter,  or  instrument  for  measuring  heathy 
melting  of  ice.  In  such  case  there  would  be  a  constant 
stream  of  water  from  the  instrument,  indicating  a  con- 
stant evolution  of  heat  generated  by  the  combustion  of 
waste.  Furthermore,  the  formation  of  water,  and  there- 
fore the  generation  of  heat,  would  be  in  proportion  to 
the  grade  of  life  and  the  amount  of  activity.  All  this  is 
expressed  by  formula  3  : 

3.   Heat   oc   Waste   oc   Life  a    Work. 

4.  Supply. — Now  it  is  evident  that  this  can  not  go  on 
continuously  without  a  contrary  process,  otherwise  the 
body  would  completely  consume  itself  and  exhale  in 
smoke,  like  the  cigar.  There  must  be  a  supply  exactly 
proportioned  to  the  7vaste.  It  is  this  that  creates  the  neces- 
sity iox  food.     Food,  therefore,  must  be  proportioned  to 


NUTRITIVE    FUNCTIONS.  285 

waste,  and  therefore   to   grade  of   life   and  intensity  of 
work.     This  is  expressed  in  formula  4: 

4.   Food  oc    IWiste   oc    Life  and  Work. 

Therefore  life  in  a  scale  pan  would  show  a  continual 
oscillation  of  level — i.  e.,  of  weight  of  body.  In  adults, 
whose  supply  is  only  equal  to  waste,  the  average  weight 
is  maintained  ;  in  a  child  the  supply  is  a  little  greater 
than  the  waste,  and  the  average  weight  increases. 

Observe  that  the  most  fundamental  of  these  two  op- 
posite processes  is  the  waste.  This  is  continuous  with 
life  and  apparently  its  cause;  the  other  (supply)  is  oc- 
casional. The  waste  goes  on  continuously,  whether 
there  be  supply  or  not,  as  long  as  life  lasts.  The  sup- 
ply may  be  regarded  as  a  secondary  consequence  of 
the  waste. 

Illustrations. —  i.  Thus  the  living  animal  body  may  be 
compared  to  a  burning  lamp — ever  consuming  and  ever 
resupplied.  In  both  cases  there  is  waste  and  supply, 
and  in  both  cases  there  is  continual  oscillation  of  weight. 
In  both  cases  heat  is  produced  by  the  consumption  of 
material.  Moreover,  the  amount  of  heat  produced  by 
the  burning  of  a  pound  of  material  is  substantially  the 
same  in  the  two  cases,  only  in  the  one  case  the  heat  is 
concentrated  on  a  given  point  and  compressed  into  a 
short  time,  and  is  therefore  intense,  while  in  the  other  it 
is  spread  over  the  whole  body  and  stretched  over  twenty- 
four  hours,  and  is  therefore  less  intense  at  one  time  and 
place. 

2.  Again,  the  living  body  may  be  compared  to  a 
temple  on  which  are  constantly  engaged  two  opposite 
forces — the  one  tearing  down,  the  other  repairing;  the 
one  rtVstructive,  the  other  r^//structive ;  the  more  rapid 
the  destruction,  the  more  active  the  construction.  These 
two  go  on   with  varying  success  until   at  last    the  de- 


286   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

structive  forces  prevail  and  the  struggle  terminates  in 
death. 

3.  Or,  again,  the  living  body  may  be  compared  to  a 
pool,  with  its  inlet  and  outlet.  The  form  remains  the 
same,  but  the  matter  is  continually  changing.  The  more 
rapid  the  change,  the  quicker  and  fresher  is  the  water. 

Waste  Removal. — It  is  seen,  then,  that  one  necessity 
arising  from  waste  is  food.  But  there  is  another  and 
far  more  urgent  necessity — viz.,  the  quick  removal  of 
7vaste  from  the  body.  The  reason  is  that  the  waste  is 
poisonous  to  the  blood.  Food-taking  may  be  delayed  for 
a  day  or  several  days,  or  even  perhaps  for  forty  days; 
but  waste  removal,  suspended  for  five  to  ten  minutes, 
destroys  life. 

For  the  removal  of  waste  there  are  two  main  pipes — 
viz.,  the  lungs  and  the  kidneys.  In  the  one  case  the  re- 
moval is  by  combustion,  in  the  other  by  solution.  In  the 
one  the  final  product  is  gaseous,  in  the  other  liquid. 
By  far  the  largest  amount — seven  eighths  of  the  whole* 
— is  removed  by  the  lungs,  and  the  urgency  of  this  re- 
moval is  also  the  greatest.  Stop  the  removal  by  the 
lungs,  and  death  occurs  in  five  minutes;  stop  the  elimi- 
nation by  the  kidneys,  and  death  occurs  by  blood 
poisoning  in  about  forty-eight  hours. 

Thus,  to  summarize,  there  are  going  on  continually 
in  the  living  body  two  opposite  processes— the  one  gath- 
ering and  constructive,  the  other  destructive  and  removing; 
the  one  ascensive  from  food  to  tissue,  the  other  descensive 
from  tissue  to  ivaste  removed.  The  one  is  called  anabo- 
lism,  the  other  katabolistn.  The  one  is  nutrition  proper, 
the  other  is  decomposition  and  elimination.  The  latter — 
i.  e.,  katabolism — is  that  which  is  most  closely  connected 
with  life;  it  is  that  which  starts  the  whole  process,  that 

*  Berthelot,  Rev.  Sci.,  viii,  134,  1S97. 


NUTRITIVE    FUNCTIONS.  287 

which  generates  life  force  itself.  Even  the  force  neces- 
sary for  anabolism  seems  to  be  generated  by  katabolism. 
The  truth  of  this  proposition  we  are  not  yet  prepared  to 
prove,  but  will  do  so  hereafter. 

The  whole  subject  of  the  nutritive  functions — i.  e., 
all  the  functions  concerned  in  the  conservation  of  the 
life  and  health  of  the  individual — divides  itself  naturally 
into  three  parts,  viz.,  ascensive,  distributive,  and  descensive. 
The  first  is  anabolic,  the  second  intermediate,  and  the 
third  katobolic.  The  first  includes  all  the  processes 
from  crude  food  to  finished  tissue  ;  the  third,  all  the  pro- 
cesses from  perfect  tissue  through  all  its  changes  by 
decomposition  to  final  elimination  from  the  body  as 
waste ;  the  second,  or  intermediate,  all  the  processes 
whereby  food  is  distributed  to  all  parts  of  the  body  and 
also  all  waste  is  distributed,  each  kind  to  its  appropriate 
organ  of  elimination.  Briefly,  they  may  be  called  food 
preparation,  food  and  waste  distribution,  and  waste  re- 
moval. Each  is  concerned  with  a  distinct  system  of  or- 
gans— the  first  with  the  digestive  system,  the  second  with 
the  blood  system,  and  the  third  with  the  excretory  system. 


CHAPTER   II. 

NUTRITION    PROPER ANABOLISM FOOD    PREPARATION 

— DIGESTIVE    SYSTEM. 

This  includes  all  the  changes  from  crude /^<?^  to  fin- 
ished tissue. 

SECTION    I. 
Food :  its  Kinds  and  Uses. 

Definition. — The  word  food  is  used  in  a  wider  and  a 
narrower  sense.  In  the  wider  sense  it  includes  all  sub- 
stances the  ingestion  of  which  is  necessary  to  life.  In 
this  sense  it  includes  water  and  air  and  many  salts.  In 
a  narrower  sense  it  means  such  substances  as  are  used 
for  tissue  building  and  iov  force  making.  It  is  in  the  nar- 
row sense  that  we  shall  use  it  here. 

Kinds. — In  this  sense  there  are  three  kinds  of  food, 
viz.,  albuminoids,  amyloids,  and/a/i-.  The  first  are  com- 
posed of  C,  H,  O,  N,  and  sometimes  a  little  P  and  S,  and 
are  therefore  called  quaternary  compounds,  or  often  ni- 
trogenous compounds;  the  second,  of  C,  H,  and  O,  the 
two  latter  in  proportions  forming  water,  and  are  there- 
fore called  carbohydrates ;  and  the  third,  also  of  C,  H, 
and  O,  but  the  C  in  excess  and  the  O  in  deficit.  The 
last  two  are  called  ternary  compounds.  Examples  of 
the  first  are  found  in  albumen  (white  of  egg),  fibrin  (lean 
meat),  gluten  of  wheat  and  other  grains,  legumin  of 
peas,  beans,  etc.,  and  protoplasm,  or  living  substance  of 
288 


NUTRITION    PROPER. 


289 


animals  and  plants.  Examples  of  the  second  are  the 
sugars  and  starches,  and  of  the  third  all  the  animal  and 
vegetable  fats  and  oils.  The  following  schedule  ex- 
presses most  of  these  facts  : 


Name. 


Food 


f  Albuminoids 
■I  Amyloids. . . . 
I  Fats 


Composition. 


j  C,  H,  O,  N,  etc.,  Quater- 
(      nai-y 

C,  H,  O,  Ternary 

+         - 

C,  H,  O,  Ternary 


Examples. 


{Fibrin,  albumen, 
protein,     gluten, 
protoplasm. 
Starches  and  sugars 

Fats  and  oils. 


Milk. — The  material  prepared  by  Nature  as  the  food 
of  the  young  of  mammals  must  contain  all  these,  .\lbu- 
minoids  are  represented  by  the  casein,  or  curd  ;  amyloids, 
by  the  milk-sugar;   and  fats  by  the  butter. 

Uses  of  Food. — The  uses  of  food  are  twofold,  viz., 
(i)  for  tissue-building — i.e.,  repair  of  waste  in  the  adult 
and  repair  and  growth  in  the  young;  {2)  iov  force  and 
heat  making  by  combustion,  or,  w*e  may  say  briefly,  tissue 
food  and/?/^/  food.     This  is  expressed  by  schedule: 


Food 


Tissue. 
Fuel.  .  , 


I  Repair. 
(  Growth. 
j  Force. 
/  Heat. 


We  say  fuel  for  force  and  /leat,  but  the  real  ob- 
ject in  the  animal  body,  as  in  the  steam  engine,  is 
force,  although,  in  both,  heat  is  a  necessary  concomi- 
tant. In  the  case  of  the  body  it  is  sometimes  an  in- 
different concomitant,  as  in  a  moderate  temperature  ; 
sometimes  a  very  comfortable  concomitant,  as  in  cold 
weather;  and  sometimes  a  distressing  concomitant,  as 
in  very  hot  weather.  We  will  explain  this  more  fully 
hereafter. 


290 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 


Distinctive  Uses  of  the  Kinds. — The  albuminoids 
are  used  mainly  for  tissue-building,  and  they  alone  can 
be  used  for  this  purpose,  for  the  tissues  are  themselves 
albuminoid.  But  whatever  of  albuminoids  is  left  over 
from  tissue-building  is  used  for  fuel  also — i.  e.,  for  force 
and  heat  making.  The  amyloids  and  fats  can  be  used 
only  for  force  and  heat,  for  these  do  not  contain  the 
nitrogen  necessary  for  the  formation  of  tissue.  Thus 
an  animal  fed  on  amyloids  and  fats  alone  can  not  con- 
tinue to  live  indelinitely  for  want  of  the  necessary  repair 
of  the  tissues.  This  is  called  nitrogen  starvatio?i.  There- 
fore tissue-making  can  be  done  only  by  albuminoids, 
but  force  and  heat  making  by  all  three  kinds. 

Illustrations. — Carnivores  fed  on  lean  meats  con- 
sume only  albuminoids.  They  use  first  whatever  is 
necessary  for  repair  of  tissues,  and  whatever  is  left 
over  is  burned  for  force  and  heat.  Hertjivores  take 
albuminoids  in,  relatively,  small  quantity;  their  food  is 
mostly  amyloids.  They  probably  use  the  whole  of  their 
albuminoids  for  repair,  ^and  their  amyloids  for  force  and 
heat.  Alan  is  an  oninivore,  but,  except  the  livers  on  rice 
or  potatoes,  he  probably  takes  more  albuminoid  than 
is  necessary  for  repair.  What  is  left  over,  which  we 
shall  call  albuminoid  excess,  he  uses  for  force  and  heat. 
All  the  amyloids  and  fats  are  used  for  the  latter 
purpose. 

Waste  Tissue. — But  the  waste  is  not  wasted.  This 
also  is  burned  as  fuel.  But  since  in  the  adult  the  ^vaste 
is  exactly  equal  to  the  repair,  it  is  evident  that  the 
equivalent  of  the  whole*  albuminoid  food  is  burned  as 
fuel  for  force.  But  since  amyloid  and  fats  are  also 
burned,  it  is  evident  that  the  equivalent  of  the  whole 
food  is  burned  for  force  and  heat. 

*  Except  an  incombustible  part,  as  explained  hereafter. 


NUTRITION    PROPER. 


PREPARATION    OF    FOOD. 


291 


Food  must  he  /prepared  bei ore  it  can  be  absorbed  into 
the  blood,  because  it  is  nearly  always  solid  and  must 
be  reduced  to  a  liquid  condition  before  it  can  be  taken 
up — it  must  be  able  to  soak*  through  membranes.  The 
food  of  plants  is  already  dissolved,  as  gases  in  the  air 
bathing  the  leaves,  or  as  liquids  bathing  the  roots.  It 
is  therefore  absorbed  at  once  without  further  prepara- 
tion. It  is  already  prepared  by  Nature.  But  in  animals 
there  must  be  a  reservoir  in  which  the  solid  food  is 
stored  and  dissolved.  This  reservoir  is  the  stomach,  and 
the  process  of  solution  is  digestion.  But  if  we  compare 
the  lower  with  the  higher  animals,  we  find,  in  accordance 
with  the  law  of  differentiation,  a  gradually  increasing 
complexity  in  the  process.  In  the  lowest  protozoa — 
amoeba — the  living  protoplasm  flows  around  the  prey, 
dissolves,  and  appropriates  it  at  once  into  the  tissues. 
The  captured  prey  passes  at  o>ie  step  from  crude  food  to 
living  tissue,  but  the  tissue  thus  summarily  made  is  a 
poor  article.  In  the  highest  animals,  on  the  contrary,  this 
simple  process  is  differentiated  into  many  consecutive 
processes,  and  the  finished  article  of  tissue  is  far  more 
perfect.     In  man  and  the  higher  animals  the  steps  are : 


Mechanical  process,  i     Ciiemical  process. 


1.  Mouth  digestion Insalivation.  1  Saccharization. 

2.  Stomach  digestion Chymification.      I  Peptonization. 

3.  Intestinal  digestion Chylification.         '  Emulsification. 


4.  Absorption By  capillaries  and  lacteals. 

5.  Sanguification By  liver  and  mesenteric  glands. 

6.  Circulation I 

7.  Assimilation [ 

*  We  say  soak,  but  it  is  necessary  to  remember  that  absorp- 
tion is  not  a  purely ///_;'j7V(7/ process,  for  it  is  selective. 


292 


PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


Our  plan  will  be  to  take  up  each  of  these  and  carry 
the  process  through  the  vertebrates ;  and  then  in  the 
invertebrates  to  take  up  the  whole  process  together  in 
each  department. 


SECTION    II. 

Mouth  Digestion  in    Vertebrates. 

This  includes  the  gathering  (prehension),  the  mastica- 
tion, and  the  insalivation  of  food.  Prehension  in  man, 
monkeys,  and  some  other  mammals  is  done  by  the  hands 
or  paws.     In  some,  as  the  elephant,  by  the  snout,  but 

in  most  vertebrates 
directly  by  the 
teeth.  The  object 
oi mastication  is  trit- 
uration  of  the  food 
for  more  perfect 
insalivation,  and  is 
done  by  the  teeth. 
We  shall  have 
much  to  say  about 
this  process  in  con- 
nection with  the 
comparative  mor- 
phology of  the 
teeth  and  physiol- 
ogy of  mouth  di- 
gestion. For  the 
present  we  dwell 
only  on  the  process  of  insalivation  and  its  effect  on 
digestion  of  the  food. 

Salivary  Glands. — There  are  three  pairs  of  these, 
viz.,  the  parotids,  p  (Fig.  178),  on  the  side  of  the  face 
just  below  and  a  little  in  front  of  the  ear;  the  submax- 


FiG.  17S. — Lower  jaw  on  right  side  and  some 
adjacent  parts  cut  away  so  as  to  show  the 
salivary  glands  :  p,  parotid  ;  sm,  submaxil- 
lary;  i-/,  sublingual.     (After  Cleland.) 


NUTRITION    PROPER. 


293 


illaries,  sm,  just  within  the  angle  of  the  lower  jaw  on 
each  side;  and  the  sublinguals,  si,  just  behind  the  chin 
on  each  side.  Every  gland  has  its  excretory  duct.  That 
of  the  parotids  runs  forward  and  opens  into  the  mouth 
between  the  cheek  and  the  upper  jaw  teeth  on  each 
side;  that  of  the  submaxillaries  runs  upward  and  opens 
on  each  side  of  the  back  part  of  the  tongue;  while  that 
of  the  sublingual  opens  on  each  side  of  the  frenum  near 
the  tip  of  the  tongue. 

Structure. — Imagine  a  slender  tube  the  size  of  a 
knitting  needle  branching  and  rebranching  to  capillary 
fineness,  each  capillary  branch  terminating  in  a  saccule, 

ep 


Fig.  179. — Structure  of  a  gland  :  i,  2,  3,  4,  different  stages  in  the  process  of 
infolding  of  the  epithelial  surface,  ep  ;  4,  fully  formed  gland. 


the  whole,  even  to  the  minutest  branch  and  saccule, 
lined  with  a  pavement  of  living  nucleated  cells,  an  ex- 
tension of  the  epithelium  which  lines  the  mouth  cavity, 
then  the  whole  system  of  tubes  webbed  together  by 
loose  connective  tissue  and  invested  with  a  membrane 
of  fibrous,  or  condensed  connective  tissue,  and  we  have 
a  tolerably  good  general  idea  of  the  structure  of  a  sali- 
vary gland,  which  may  indeed  be  taken  as  a  type  of  a 


294 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


secreting  gland.  It  is  evidently  only  a  device  to  bring 
as  large  a  surface  as  possible  of  epithelial  cells  into  a 
small  space;  for  the  secretion  is  a  product  of  epithe- 
lial action.  Fig.  179  shows  the  different  stages  of  in- 
folding. 

Hxcitant. — Any  stimulation  of  the  tongue  and  in- 
terior of  the  mouth,  even  mechanical,  will  excite  the 
secretion,  but  especially  any  strong  taste.  Even  the 
sight  of  food  or  the  idea  of  food  will  often  cause  it  to 
flow.  The  first  effect  of  excitement  is  the  rush  of  blood 
to  the  gland,  and  then  follows  a  flow  of  secretion.  Evi- 
dently the  secretion  is  manufactured  out  of  materials  in 
the  blood.  This  preliminary  rush  of  blood  and  conse- 
quent swelling  of  the  gland  is  the  cause  of  the  pain  pro- 
duced by  food  or  even  the  sight  of  food  in  mumps,  which 
is  a  disease  of  the  parotid  gland. 

Composition  and  Use  of  Saliva. — It  is  easy  to 
collect  the  salivary  secretion  in  considerable  quantity, 
and  thus  to  determine  its  composition.  In  a  horse  the 
duct  of  the  parotid  gland  runs  just  beneath  the  skin 
over  the  broad,  flat  surface  of  the  jaw  and  may  be  easily 
taken  up,  a  metallic  tube  introduced  and  turned  out- 
ward. If,  now,  food  be  given,  or  even  a  sheaf  of  hay  be 
shown,  immediately  the  liquid  secretion  begins  to  pour 
from  the  tube,  and  continues  to  pour  as  long  as  the  food 
is  masticated.  In  this  way  half  a  pint  or  a  pint  of  the 
clear  liquid  may  be  collected. 

Saliva  is  a  watery  liquid,  consisting  mainly  of  mucus 
(water  and  broken-down  epithelial  cells),  but  containing 
a  peculiar  ferment  called //jy///;;,  which  is  its  active  prin- 
ciple ;  its  composition  and  its  chemical  properties  seem  to 
be  identical  with  diastase  of  sprouting  seeds.  Like  dias- 
tase, it  changes  the  insoluble  forms  of  amyloids  (starch) 
into  the  soluble  forms  (sugar).  It  does  so  by  hydra- 
tion of  the  starch. 


NUTRITION    PROPER.  295 


Starch.  Water.  .Sugar  (glucose). 

QH.0O3     +     H,0     =     QH„0, 

Its  function,  then,  is  the  digestion  of  amyloids.  This, 
of  course,  takes  time.  Starches,  therefore,  are  dissolved 
in  the  stomach,  although  the  digestive  juice  is  made  in 
the  mouth.  Ptyalin  is  far  more  important  in  herbivores 
than  in  carnivores. 

Ferments. — Ferments  are  of  two  general  kinds — viz., 
those,  like  yeast,  that  contain  living  microbes  which  de- 
termine decomposition  in  the  fermentmg  substance,  and 
those,  like  diastase,  that  contain  no  microbes  and  deter- 
mine change,  but  not  decomposition.  All  the  digestive 
ferments  are  of  this  latter  kind.  They  are  called  en- 
zymes. 

After  mastication  and  thorough  insalivation  the  food 
is  gathered  into  a  bolus,  pressed  by  the  tongue  into 
the  throat,  and  swallotved — i.  e.,  it  is  there  seized  by 
the  involuntary  muscles  and  hurried  into  the  stomach. 
There  we  leave  it  for  the  present  to  take  up  the 

COMP.ARATIVE    PHYSIOLOGY    OF    MOUTH    DIGESTION    IN 
VERTEBRATES. 

The  chemical  process  of  saccharization  is  precise- 
ly the  same  in  all  vertebrates  and  probably  in  all 
animals.  The  only  important  variation  is  in  the  me- 
chanical processes  of  food-taking  and  mastication,  espe- 
cially the  latter.  This  brings  us  to  the  important 
subject  of 

Teeth  invertebrates. — We  take  up  this  somewhat 
fully  on  account  of  its  important  bearing  on  classifica- 
tion, especially  of  mammals.  The  character  of  the  teeth 
is  determined  by  the  food,  and  the  nature  of  the  food 
determines  the  habits,  and  therefore  the  whole  structure 
of  the  animal.  All  the  parts  of  an  animal  are  in  har- 
monic relation  with   one  another.     The  keynote  of  this 


296   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

complex  harmony  is  the  teeth.     Next  to  the  teeth,  foot 
structure  is  most  important  in  classification. 

Mammalian  Teeth. — i.  Origin  and  Development. — In 
speaking  of  the  skeleton  we  said  (page  224)  that  teeth 
do   not  belong  to  the  true  internal  skeleton,  but  are  an 


Fig.  180. — I,  2,  3,  4,  5,  6,  7,  showing  successive  stages  in  the  development 
of  a  human  incisor  :  //,  tooth  pulp  ;  ts,  tooth  sac. 


epidermal  structure,  or,  more  specifically,  an  epithelial, 
a  gum  structure.  The  evidence  of  this  is  found  in  their 
embryonic  development.     The    following   figures   (Fig. 


NUTRITION    PROPER.  297 

180,  I,  2,  3,  etc.)  show  the  embryonic  development  of 
teeth  in  man.  The  same  process  is  true  of  all  mammals. 
At  first  there  is  a  little  papilla  on  the  gum,  which  is 
the  pulp  of  the  future  tooth  (i).  Then  this  is  sunk  more 
and  more  into  the  gum,  to  become  the  future  socket  (2). 
Then  the  pulp  begins  to  secrete  the  tooth,  and  the  socket 
closes  up  above,  forming  the  tooth  (sac  3,  4,  and  5),  and 
the  tooth  is  now  entirely  inclosed  in  the  gum  and  in  the 
jawbone.  Then  by  continued  growth  it  breaks  through 
the  jaw  and  the  gum  and  the  tooth  is  cut,  and  thence 
continues  to  grow  to  its  full  size  (6  and  7).  But  some 
teeth — milk  teeth — are  shed  and  replaced  by  permanent. 
Are  these  also  gum  structures  ?  Yes.  It  is  seen  that 
the  tooth  sac  has  a  saccule  on  each  side.  One  of  these 
continues  to  develop,  and  a  tooth  is  formed  in  it.  This 
grows,  and  finally  pushes  out  the  first  tooth  and  takes 
.its  place  (5,  6,  7).  In  some  rare  cases  the  other  saccule 
also  forms  a  tooth,  which  may  develop.  It  is  in  this 
way  that  we  account  for  those  rare  cases  of  a  third  set 
of  teeth.  But  what  is  exceptional  in  man  is  the  rule 
in  many  reptiles  and  in  sharks,  as  we  shall  see  here- 
after. 

2.  Composition  of  Teeth. — Mammalian  teeth  consist  usu- 
ally of  three  kinds  of  substance — viz.,  dentine,  enamel,  and 
cement.  The  dentine  is  a  denser  kind  of  bone,  already 
described,  and  forms  the  principal  part ;  the  enamel  is  a 
still  denser  variety,  and  covers  the  crown  or  exposed 
part ;  and  the  whole  is  covered  and  the  inequalities 
filled  up  with  cement,  which  is  a  more  structureless 
variety  than  either.  The  cement  is  almost  wanting  in 
many  teeth,  as  man's,  and  in  such  cases  is  quickly  worn 
off  and  disappears,  but  is  an  important  part  of  the  more 
specialized  teeth  of  many  mammals. 

3.  Kinds  of  Teeth. — There  are  four  kinds  of  teeth  in 
the   jaws  of   mammals — viz.,  (i)   the   incisors,   or  front 


298 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


teeth,  (2)  the  canines  (tusks  of  carnivores,  eye  teeth  of 
man),  (3) //rwtp/rtrj-,  or  deciduous  molars  or  bicuspids  of 
man,  and  (4)  the  permanent  or  true  molars.  The  func- 
tion of  the  first  is  the  cutting  off  of  morsels  of  food,  of 
the  second  is  seizing  and  holding  the  prey.  The  jaw  teeth 
— i.  e.,  the  molars  and  premolars — are  the  true  masticat- 
ing teeth.  This  grouping  of  the  teeth  into  kinds  having 
different  functions  is  very  characteristic  of  mammals. 

4.  Variation  of  the  Teeth. — The  teeth  of  different  or- 
ders and  families  of  mammals  vary  in  relative  size  and 
relative  number  of  the  kinds,  and  in  the  structure  of  the 
molars. 

(a)  Relative  Size. — In  man  all  the  teeth  are  of  similar 
size,  forming  thus  a  continuous  even  arch;  but  in  the 
more  specialized  mammals  some  of  the  teeth  may  be 
enormously  developed — for  example,  the  canines  in  car- 
nivores, and  especially  the  walrus;  the  incisors  in  ro- 
dents, and  especially  in  proboscidians  (elephants).  In 
cases  of  enormous  development,  such  as  the  incisors  of 
rodents  and  the  tusks  of  the  walrus,  the  boar,  and  the 
elephant,  the  pulp  is  permanent,  and  the  tooth  grows 
continuously.  Such  teeth  have  a  hollow  at  the  base. 
Thus,  teeth  are  sometimes  of  definite  and  sometimes  of 
indefinite  growth. 

[b)  N' umber  ofi  Teeth  and  Relative  Number  of  the  Kinds. 
— The  normal  number  of  mammalian  teeth  seems  to  be 
forty-four,  and  any  less  number  must  be  regarded  as  the 
result  of  gradual  loss;  precisely  as  in  the  case  of  toes 
of  less  number  than  the  normal  five.  But  the  whole 
number,  and  especially  the  relative  number  of  the  sev- 
eral kinds,  vary  in  a  way  which  is  very  characteristic 
of  the  different  orders  and  families  of  mammals.  This 
introduces  the  subject  of  the  dental  formula,  which  is  a 
compendious  way  of  expressing  the  number  of  different 
kinds  of  teeth  in  mammals,  very  necessary  in  descrip- 


NUTRITION    PROPER. 


299 


tion   of   mammals,  and   therefore   universally   used    by 
naturalists.     A  number  of  these  are  given  below  : 


Type  

i     -^"^  ■ 
3—3  ' 

c, 

I — I 
I — I  ' 

pm., 

4—4. 
4—4' 

m. 

■3-3      ''■ 

Man 

■     2—2 
'  2—2  ' 

c, 

I — I 

I — I  ' 

pm., 

2 — 2 
2—2' 

m., 

■P=- 

Bear 

J        3     . 
1.,      ^     , 

c. 

I 
I     ' 

pm.. 

4    . 
4 

m., 

^     I     =- 

CaU 

J       3     . 
'     3     ' 

c, 

I 
I 

pm., 

3     . 
2      ' 

m., 

I 

0 

c, 

0 
I     ' 

pm., 

3    . 
3 

m., 

.-f=3. 

3 

Rodent 

2 

c, 

0 
0 

pm., 

3    . 
2     ' 

m., 

3    -"8 

'      I     ' 

3 

Sloth , 

0 
'      0 

c, 

I 
I    ' 

m., 

^    -13 

3 

00  00 

Ant-eater 1.,  ;    c.,  ; =    o. 

00  00 

Ornithorhynchus O. 

To  explain  :  In  the  type  mammal,  probably  in  all  the 
early  Tertiary  mammals  and  in  the  most  generalized 
mammals,  like  the  hog  7io7v  (Fig.  183),  there  are  forty- 
four  teeth  in  all,  of  which  twelve  are  incisors — i.  e.,  three 

on  each  side  above  and  below,  1.  = ;  four  are  ca- 

3  —  Z 

nines — i.  e.,  one   on   each   side   above  and   below,  c.  = 

;  sixteen   are   premolars — i.e.,   four  on  each  side 

I  —  I 

4  —  4 
above  and  below,  pm.  = ;  and  twelve  are  molars 

4  —  4 

•7  —  2 

— i.  e.,  three  on  each  side  above  and  below,  m.  = —. 

But,  as  the  bilateral  symmetry  is  always  perfect  in  the 
teeth,  there  is  no  necessity  to  express  but  one  side — i.  e., 
all  the  teeth  as  seen  from  one  side,  as  in  all  the  formulae, 
except  the  first  two. 

The   normal    number,  we    have    said,   is    forty-four. 


300 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


This  was  probably  the  number  in  the  early  Tertiary 
mammals  ;  any  less  number  is  the  result  of  gradual  abor- 
tion. Thus  ruminants  have  no  front  teeth  in  the  upper 
jaw,  but  the  rudiments  of  these  buried  in  the  jawbone 
unborn  show  that  they  came  from  animals  having  a  full 
set.  The  same  is  true,  as  already  explained  (page  260), 
of  whales,  and  also  of  edentates,  etc. 

(c)  Structure  of  Molars. — Molars  are  the  masticatory 
teeth,  and  are  therefore,  more  than  all  others,  subject 

to  variation  accord- 
ing to  the  character 
of  the  food.  In  this 
regard  there  are  three 
main  kinds,  viz.,  om- 
nivorous., carnivorous, 
and  herbivorous.  Om- 
nivorous molars  are 
simply  tuberculated, 
and  are  equally  adapt- 
ed to  all  kinds  of 
food,  but  not  specially  and  perfectly  adapted  to  any  one 
kind.  Such  are  the  teeth  of  man,  of  monkeys,  of  bears, 
and  the  hog  (Figs.  181,  182,  183). 


Fig.  181. — View  of  teeth  of  the  right  side 
of  the  upper  jaw  of  man. 


Fig.  182.— Teeth  of  the  right  side  of  the  upper  jaw  of  a  monkey. 


Carnivorous  molars  are  specially  adapted  for  flesh 
eating.  They  only  crush2iX\6.  divide  the  food  sufficiently 
for  swallowing,  but  do  not  grind  or  triturate  it.     This  is 


NUTRITION    PROPER. 


301 


not  necessary,  because  thorough  insalivation  is  not  re- 
quired for  flesh-food  (Fig.  184). 

Herbivorous  molars  are  by  far  the  most  specialized 
and  complex,  because  their  food  requires  the  most  com- 
plete trituration  and  insalivation  (Fig.  185). 


Fig.  183. — Teeth  of  the  right  side  of  the  upper  jaw  of  a  hog. 

Undoubtedly  the    primal    mammal   was   omnivorous 
and  had  simple  tuberculated  molars.     From  this  gener- 


FiG.  184. — Side  view  of  the  upper  jaw  of  a  dog. 

alized  form,  as  time  went  on,  mammals  were  specialized 
in  two  main  directions — the  one  more  and  more  adapted 


Fig.  185. — Face  view  of  the  upper  jaw  of  a  sheep. 

to  flesh  eating,  the  other  to  herb  eating.  The  extreme 
forms  are  represented  now  by  the  cat  tribe  on  the  one 
hand  and  the  ruminants  on  the  other.     In  the  meanwhile 


302    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


an  intermediate  type   continued  in  a  more  generalized 
form,  including  man.     The  diagram  shows  these  facts: 

/Carnivorous. 


Omnivorousc  .  .  .  Monkey,  man,  etc. 
^  Herbivorous. 

Structure  of  Herbivorous  Molars. — The  most 
interesting  examples  of  masticatory  teeth  are  found 
in  herbivores.  In  these  we  have  veritable  upper  and 
nether  millstones.  Fig.  i86  is  a  face  view  of  a  molar 
of  a  horse.  The  double  lines  are  enamel,  the  shaded 
spaces  dentine,  and  the  unshaded  cement.  On  account 
of  its  greater  hardness,  the  enamel  stands  out  as  ridges 
above  the  softer  dentine  and  cement,  and  continues  to 
do  so  however  much  the  tooth  wears. 


Fig.  i86. — Grinding;  face  of 
a  horse's  molar. 


Fig.  187. — Incisor  of  a  horse  : 
A,  vertical ;  B,  cross  section. 


Origin  of  this  Structure. — The  dentine  is  secreted 
by  the  tooth  pulp,  the  enamel  by  the  membranes  of  the 
tooth  sac.  Therefore  the  tooth  sac  must  have  followed 
the  enamel  in  all  its  windings.  The  complexity  of 
structure  is  therefore  the  result  of  tnfoidi/igs  of  the  sac 
on  the  side  and  down- dippings  of  the  same  from  above. 
The  cement  is  afterward  formed  on  the  outside,  and,  as 
it  were,  poured   over  all,  filling  up  the  inequalities.     A 


NUTRITION    PROPER. 


303 


simple  case  of  the  down-dipping  is  seen  in  the  front 
teeth  of  the  horse.  In  this  case  the  down-dipping  (Fig. 
187,  A)  determines  on  cross  section  a  concentric  arrange- 
ment of  the  enamel  and  cement  (Fig.  187,  B),  which  suc- 
cessively disappear  as,  by  use,  the  tooth  wears  to  lower 
and  lower  level  [a  l>,  c  d,  ef),  first 
the  cement,  and  then  the  enamel, 
until  only  the  dentine  remains. 
These  changes  occur  first  in  the 
middle  front  teeth,  and  then  in  the 
side  teeth.  On  this  fact  is  founded 
the  mode  of  estimating  the  age  of 
horses  by  the  teeth. 

In  the  horse  and  cow  the  ar- 
rangement of  the  enamel  plates 
among  the  dentine  and  cement  is 
adapted  for  side-to-side  grinding, 
but  some  mammals  have  fore-and- 
aft  grinding.     In  these  the  enamel 

plates  are  transverse.     This  is  well  seen  in  rodents  (Fig. 
188),  and  especially  in  the  elephant.     The  molar  of  an 


Fig.  188.— The  grinding 
face  of  a  molar  of  a  paca 
( Caelos^enys)  of  South 
America,  enlarged. 


Fig.  189. — Elephant's  molar  :  A,  showing  side  and  grinding  face  ;  B,  section 
showing  the  plates  ;  s  .f,  original  surface  ;  a  b,  worn  to  a  face  as  in  A. 


304 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


elephant  is  the  most  efficient  fore-and-aft  grinder  imagi- 
nable. The  whole  tooth,  as  seen  (Fig.  189),  is  composed 
of  narrow  transverse  islands  of  dentine,  surrounded  by 
ridges  or  plates  of  enamel  separated  by  cement.  The 
section  shows  how^  this  has  been  formed.  Imagine  a 
mass  of  dentine  from  which  spring  many  thin  plates  of 
the  same,  each  plate  sheathed  with  enamel,  and  then 
cement  poured  over  the  w'hole,  and  finally  the  tooth 
subjected  to  wear. 

Mouth  Armature  of  Whales. — Many  cetaceans — 
e.  g.,  the  sperm  whales,  porpoises,  etc. — have  teeth,  but 
these  are  conical,  prehensile,  not  masticatory,  teeth. 
But  the  baleen  (whalebone)  whales  have  no  teeth,  but 
in  their  stead  have  the  most  efficient  food-taking  ap- 
paratus known.     These   animals  have  enormous   heads 


A  B 

Fig.  190. — Head  of  a  whale  :  A,  side  view  ;  B,  section. 

(nearly  half  the  whole  body),  fifteen  to  twenty  feet  long 
and  ten  to  fifteen  feet  deep,  and  nearly  the  whole  of  this 
great  head  is  mouth.  This  huge  cavern  is  largely  occu- 
pied with  whalebone  plates  (Fig.  190). 

These  horny  plates,  hundreds  in  number,  are  at- 
tached above  to  the  roof  of  the  mouth,  hang  down, 
and  are  split  up  into  fibers  at  their  edges,  so  that 
the  open  mouth  is  like  a  moss-roofed  cavern.  The 
hollow  space  beneath  is  filled  up  by  the  enormous 
tongue. 


NUTRITION    PROPER. 


305 


Mode  of  Feeding. — The  baleen  whales  feed  on 
squids,  cuttlefish,  medusae,  and  small  crustaceans,  which 
exist  in  enormous  numbers  in  arctic  seas,  usually  near 
the  surface.  The  whale  rushes  forward  at  great  speed 
with  mouth  open,  so  the  water  pours  like  a  torrent  into 
the  mouth  and  out  at  the  sides  between  the  plates.  All 
the  surface  animals  are  caught  on  the  mossy  roof  and 
sides.  When  a  sufficient  quantity  is  gathered  the  mouth 
is  closed,  the  superfluous  water  is  spouted  through  the 
blowhole  (nostril),  and  the  prey  is  swept  up  by  the 
tongue  and  swallowed. 

Homology  of  Baleen  Plates. — These  plates  are  a 
substitute  for,  not  a  modification  of,  teeth.  They  are 
therefore  analogous,  but  not  homologous  with  teeth.  As 
already  explained,  these 
whales  have  rudiments 
of  teeth,  which  are  nev- 
er cut.  What,  then,  are 
the  plates  homologous 
with  ?  They  are  prob- 
ably extreme  modifica- 
tions of  gu^ii  ridges — 
such,  e.  g.,  as  those  found 
on  the  mouth-roof  of  a 
horse.  If  each  of  such 
ridges  produced  a  down- 
ward growth  of  horny 
tissue  we  should  have 
something  like  the  ba- 
leen plates  of  the  whale. 

Birds.  —  Existing 
birds  have  no  teeth,  yet 
in  the   embryo  of  some 

birds    rudimentary    teeth    are    found    which    are    never 
developed.     This  is,  of  course,  strong  presumptive  evi- 


FiG.  195. — Ichthyornis  victor,  x  J^. 
(^Restored  by  Marsh.) 


3o6  PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 

dence  that  birds  once  had  teeth,  but  they  have  gradually- 
dwindled  and  passed  away,  because  another  apparatus 
— viz.,  a  horny  beak — was  used  in  its  place.  This  pre- 
sumption is  confirmed  by  the  finding  of  Jurassic  and 
Cretaceous  birds  with  the  mouth  full  of  teeth — true  sock- 
eted \.^&\.\i  (Fig.  191).  These,  however,  were  conical,  pre- 
hensile, not  masticatory  teeth. 

Reptiles. — There  is  great  variety  among  reptiles  in 
this  regard.  This  was  to  be  expected,  for  they  are  the 
ancestors  of  both  birds  and  mammals.  Some,  as  the  tur- 
tles, have  no  teeth,  but,  like  birds,  a  horny,  nipping  beak 
in  their  place.  Some,  like  serpents,  have  teeth  not  only 
in  the  jaws,  but  on  other  bones  of  the  mouth,  as,  e.  g., 
the  palatal.  But  in  all  reptiles  which  have  teeth  these 
are  conical,  prehensile,  and  not  masticatory  teeth.  Also, 
with  the  exception  of  crocodilians,  the  teeth  are  not  sock- 
eted. They  are  formed  in  a  fold  of  the  gum,  and  after- 
ward fixed  to,  but  not  sunk  into  and  inclosed  by  the 
jawbone.  Among  extinct  reptiles  socketed  teeth  were 
more  common.  In  serpents  the  teeth  all  point  back- 
ward. This  is  necessary  in  swallowing  large  prey,  which, 
by  a  peculiar  movableness  of  the  bones  of  the  head,  are 
thus  dragged  by  main  force  down  the  throat. 

Fangs  of  Serpents. — These  are  worthy  of  brief 
notice  as  an  example  of  admirable  adaptive  modification. 
First,  observe  that  the  canal  in  the  tooth  which  conveys 
the  poison  does  not  go  to  the  end,  for  that  would  inter- 
fere with  the  keenness  of  the  point,  but  comes  out  a  lit- 
tle short  of  the  extreme  end.  The  same  device  is  used 
in  the  subcutaneous  injector  of  the  surgeon.  Observe, 
second,  that  the  formation  of  the  poison  canal  is  not  in 
violation  of  the  ordinary  structure  of  teeth,  but  a  curi- 
ous modification  of  it.  It  is  not  along  the  tooth  cavity, 
but  is  really  outside  of  the  tooth.  Fig.  192,  a,  represents 
a  section  of  a  flat  tooth.     Suppose  such  a  tooth  bent 


NUTRITION    PROPER 

upward  (^  and  c)  until  the 
edges  meet  above  and  are  sol- 
dered ;  //  is  the  poison  tube. 
That  this  peculiar  structure 
was  gradually  formed  is 
shown  by  the  fact  that  all 
the  steps  of  the  process  may  pf- 
be  found  among  living  ser- 
pents—  viz.,  teeth  slightly 
grooved,  deeply  grooved, 
and  tubulated.  All  degrees 
of  virulence  of  the  poison 
may  be  found.  The  poison 
gland  is  probably  an  extreme 
modification  of  a  salivary 
gland,  and  the  poison  of 
saliva.  The  saliva  is  slightly 
poisonous  in  many  animals. 

Observe  again,  t/ii'rd,  that 
the  fang  is  formed  in  a  fold 
in  the  gum,  and  at  first  loose, 
but  afterward  fi.xed  to  the  jawbone.     Also  that  in  the 
fold  there  are  many  subordinate  folds,  in  each  of  which 


Fig.  192. — A  longitudinal  section 
of  the  fang  of  a  serpent  :  the 
cross  sections  show  mode  of 
formation  of  the  poison  tube  ; 
//■,  poison  tube  ;  tc,  tooth  cavity  ; 
/jf,  poison  gland. 


'm 


Fig.  193. -Head  of  a  rattlesnake,  showing^  several  fangs  in  different  stages 
of  development.     The  arrow  shows  the  poison  tube  ;  m,  ma.xillary  bone. 


3o8    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 

is  formed  a  tooth,  and  thus  there  is  a  magazine  of  teeth 
of  all  sizes,  which  are  successively  brought  forward  and 
attached  as  the  old  tooth  drops  off  (Fig.  193). 

Origin  of  Mammalian  Teeth. — A  characteristic 
of  mammalian  teeth  is  that  they  are  differentiated  into 
three  groups,  distinct  in  form  and  in  function,  viz.,  incisors 
for  cutting,  canines  for  seizing,  and  molars  and  premolars 
for  masticating.  The  complex  structure  of  these  last 
is  especially  significant.  Now  in  some  early  extinct  rep- 
tiles of  the  Permian  and  Triassic  periods,  immediately 
before  the  appearance  of  mammals,  the  teeth  were  already 


Fig.  194. — Head  of  Cynog-nathus,  a  Triassic  reptile,  showing  teeth  similar 
to  those  of  mammals.     (From  Woodward.) 

differentiated  into  the  three  groups,  and  the  jaw  teeth 
were  already  become  molariform  (Fig.  194).  Indeed,  a 
complete  series  may  be  traced  from  the  simple  conical 
prehensile  teeth  of  ordinary  reptiles  to  the  most  com- 
plex grinders  of  ruminants.  It  is  certain,  therefore,  that 
mammalian  teeth  have  come  by  gradual  modification 
from  simple  prehensile  teeth  of  reptiles. 

Fishes. — The  teeth  of  fishes  are  of  three  kinds — viz., 
conical^  lancet-shaped^  q.y\<^  pavement  teeth.  The  conical  are 
the  commonest;  they  are  prehensile  only.  The  lancet- 
shaped  teeth  are  very  characteristic  of  sharks.     They  are 


NUTRITION    PROPER. 


309 


interesting  as  examples  of  magazines  of  teeth  of  all  sizes, 
and  a  successive  dropping  of  old  and  a  coming  forward 
of  new  to  their  place.  In  a  shark's  jaw  there  is  a 
magazine  of  many 
hundreds  of  teeth, 
growing  smaller  as 
we  pass  inward 
from  the  edge  of 
the  jaw  (Fig.  195). 
Only  the  large 
teeth  of  the  outer 
rows  are  in  use 
at  one  time ;  but 
there  is  a  contin- 
ual growing  outward  of  the  gum,  carrying  the  teeth  with 
it  in  the  direction  of  the  arrow.  Sharks'  teeth  are  very 
interesting   in   another   respect — viz.,   as   showing   their 


Fig.  195. — Section  of  lower  jaw  of  a  shark,  show- 
ing the  magazine  of  teeth.  The  arrows  show 
the  direction  of  replacement. 


Fig.  196. — Jaw  of  Port  Jackson  shark     Fig.  197. — Jaw  of  a  skate  (Hylo- 
(Cestraceon)^  showing  pavement  of  bates),  showing  tesselated  pave- 

rounded  teeth.     (Owen.)  ment  of  teeth.     (From  Owen.) 

homology  with  scales,  which  are,  of  course,  a  skin  struc- 
ture. Every  gradation  from  one  to  the  other  can  be 
traced  over  the  edge  of  the  jaw  of  some  sharks. 

Pavement  teeth  also  may  be  of  several  kinds,  notably 
what  might  be  called  cobble-stone  pavement   (Fig.    196) 


310  PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 

and  tessellated  pavement  (Fig.  197),  such  as  are  found  in 
skates.  Pavement  teeth  are  used  for  crushing  shells 
before  swallowing.  They  are  interesting  as  the  sim- 
plest form  of  teeth,  and  as  showing  their  origin  from 
the  gum.  They  are  a  secretion  on  the  surface  of  the 
gum,  and  afterward  fixed  to  the  subjacent  bone. 


SECTION    III. 

Stomach  Digestion  ;  Chymification  ;  Peptonization. 

As  already  seen,  after  mastication  and  insalivation, 
the  food  is  gathered  by  the  tongue  into  a  bolus  and 
pressed  into  the  throat  (so  much  is  voluntary) ;  then 
seized  by  the  involuntary  muscles  and  rushed  down 
through  the  gullet  (oesophagus)  into  the  stomach,  to 
undergo  there  the  second  stage  of  preparation.  The 
oesophagus  is  a  muscular  tube  about  ten  inches  long  and 
nearly  an  inch  in  diameter.  The  muscular  fibers  are 
mostly  circular  or  ring  fibers.  We  have  already  said 
that  a  characteristic  of  involuntary  muscles  is  a  con- 
secutive contraction  of  fibers,  producing  propagated 
waves  of  contraction  in  one  direction.  Such  a  wave  of 
contraction  propagated  downward  carries  the  food  be- 
fore it  to  the  stomach.  It  is  a  strong,  water-tight  con- 
traction, as  shown  by  the  fact  that  long-necked  animals, 
like  the  horse,  swallow  water  upward  from  the  ground 
in  drinking.  The  normal  direction  of  waves  is  down- 
ward or  stomachward.  These  are  called  peristaltic. 
Sometimes — abnormally  in  man,  as  in  vomiting,  but  nor- 
mally in  ruminants,  as  in  bringing  up  the  cud — the  waves 
may  run  in  the  contrary  direction.  These  are  called 
antiperistaltic. 

Saccharization  of  the  Food. — This  belongs  to 
mouth  digestion,  for  the  saliva  is  the  digestive  juice  for 


NUTRITION    PROPER. 


^11 


starch.  If  the  food  remained  long  enough  in  the  mouth 
the  saccharization  would  take  place  there,  but  in  fact 
it  takes  place  in  the  stomach,  although  the  stomach  has 
really  nothing  to  do  with  the  process.  On  the  contrary, 
the  gastric  juice  by  its  acidity  rather  checks  the  saccha- 
rization of  starch,  so  that  this  change  takes  place  mainly 
in  the  early  stages  of  stomach  digestion  before  gastric 
juice  is  yet  formed  in  large  quantities. 

The  Stomach. — The. positio?i  is  just  below  the  dia- 
phragm and  behind   the  triangular  space  in  front  called 


Fig.  198.  Stomach  of  man  ;  a  section  showing  form  and  interior  surface : 
oe,  cesophagus ;  /,  pylorus  ;  bd,  bile  duct ;  gbl,  gall  bladder ;  ps,  pan- 
creas. 


the  pit  of  the  stomach,  but  a  little  more  to  the  left  side. 
Its  shape  js  seen  in  Fig.  198.  Of  its  two  openings,  that 
leading  into  the  oesophagus  is  called  the  cardiac,  and 
that  leading  into  the  inte'^tino'^  \\\e pylorif  orifice. 


312    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Coats. — The  stomach  consists  of  three  coats — an 
investing  serous  coat,  very  smooth,  thin,  and  tough;  a 
middle  muscular  coat ;  and  a  lining  mucous  coat.  The  mus- 
cular coat  is  by  far  the  thickest,  so  that  the  organ  may 
be  called  a  hollow  muscle,  invested  with  serous  mem- 
brane and  Imed  with  mucous  membrane.  The  function 
of  the  serous  coat  is  to  give  toughness  and  easy  gliding 
without  friction  over  other  organs  in  the  abdomen.  The 
function  of  the  muscular  coat  is  to  do  the  mechanical  work, 
and  of  the  mucous  coat  the  chemical  work  of  digestion. 

Mechanical  Work  ;  Chymification. — The  me- 
chanical work  of  mixing  the  food  with  the  gastric  juice 
is  done  by  the  muscular  coat.  This  consists  of  several 
sheets  of  parallel  fibers  running  in  different  directions, 
longitudinal,  transverse  or  ring,  and  oblique  fibers. 
Under  the  contraction  of  these  the  stomach  is  seen  to 
squirm,  and  especially  to  transmit  light  waves  of  annu- 
lar contraction,  chasing  one  another  from  the  cardiac  to 
the  pyloric  end.  By  these  contractions  the  food  is  gen- 
tly urged  along  the.  greater  curvature  to  the  pylorus  and 
back  along  the  lesser  curvature  to  the  cardiac  orifice, 
and  so  on  repeatedly  until  the  digestion  is  complete. 

The  pylorus  (gate  keeper)  is  a  strong  collection  of 
circular  fibers  at  the  outgoing  orifice  of  the  stomach. 
During  digestion  it  is  completely  closed.  After  two  to 
five  hours,  depending  on  the  nature  of  the  food  and  the 
digestive  power  of  the  stomach,  the  digested  food  is 
allowed  to  pass  the  pylorus;  but  if  any  undigested  por- 
tions appear  the  pylorus  closes,  and  sends  it  on  its  way 
round  again  until  in  a  proper  condition.  But  sometimes, 
by  repeated  application,  the  gate  keeper  is,  as  it  were, 
teased  into  compliance,  and  even  undigested  matter  may  be 
allowed  to  pass,  and  may  create  trouble  farther  on. 
The  final  result  of  this  process  is  a  grayish,  slightly  acid, 
semiliquid  mass,  about  the  consistence  and  somewhat 


NUTRITION    PROPER. 


13 


the  appearance  of  pea  soup,  and  called  chyme.     This  is 
passed  on  to  the  intestines,  to  undergo  the  next  stage. 

Chemical  Work  ;  Peptonization. — The  gastric 
juice  is  elaborated  by  the  whole  mucous  membrane  of 
the  stomach,  but  especially  by  the  peptic  glands. 
Glands,  as  already  explained  (page  294),  are  a  device  for 
increasing  the  extent  of  the  epithelial  surface.  In  this 
case  the  device  is  of  the  simplest  sort.  The  interior  of 
the  stomach  is  thickly  strewed  with  deep///i-  lined  with 


Fig.   199. — A,    interior  of   stomach,  showing  the  openings  of  the  peptic 
glands  ;  B,  section  through  the  walls. 


epithelium  (Fig.  199).  Some  of  these  pitlike  tubes  are 
branched,  but  only  simply.  The  epithelial  surface  is 
thus  many  times  greater  than  the  interior  surface  of 
the  stomach.  The  gastric  juice  is  secreted  mainly  in 
these  pits,  both  simple  and  branched.  As  soon  as  the 
food  touches  the  stomach  the  mucous  membrane  be- 
comes engorged  with  blood  and  reddened,  and  the  gas- 
tric secretion  commences. 


314   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

Composition  and  Uses  of  Gastric  Juice. — Acci- 
dental woundings  of  the  stomach  have  afforded  means 
of  collecting  the  gastric  juice  of  man  in  considerable 
quantity  ;  that  of  animals  has  been  collected  in  vivisec- 
tion experiments.  The  secretion,  therefore,  has  been 
analyzed.  It  consists  of  mucus,  with  a  little  free  acid, 
hydrochloric  and  lactic,  and  a  peculiar  ferment,  an  en- 
zyme called/<f/j-/;;,  which  has  the  property  of  dissolving 
albuminoids.  This  it  does  by  hydration.  The  soluble 
forms  of  albuminoids  thus  formed  are  called  peptones, 
and  the  process  of  change  peptonization.  In  some 
cases  of  weak  digestion  pepsin  made  from  a  calf's  stom- 
ach may  be  used  as  medicine  with  good  effect. 

In  both  saccharization  and  peptonization  the  process 
is  purely  chemical,  and  takes  place  just  as  well  in  a  warm 
flask  as  in  the  stomach.  The  true  vital  process  is  the 
formation  of  the  ferment,  not  the  change  effected  by  it 
on  the  food. 

Effect  on  Milk. — The  effect  of  pepsin  on  milk  is 
very  characteristic.  It  first  curdles,  and  then  dissolves 
it.  The  albuminoid — casein — in  milk  is  in  a  liquid  state 
and  apparently  suitable  for  direct  absorption.  But  albu- 
minoids in  their  natural  state  are  unstable  and  liable 
to  pass  into  a  solid.  Therefore  the  casein  is  first  solidi- 
fied and  then  changed  into  peptones,  in  which  state  it  is 
no  longer  liable  to  solidification.  Advantage  is  taken 
of  this  property  of  curdling  milk  in  the  manufacture  of 
cheese.  Fresh  milk  is  treated  with  a  small  quantity  of 
an  extract  of  rennet  (calf  stomach) ;  the  quantity  used 
is  sufficient  to  curdle,  but  not  enough  to  dissolve  the 
casein. 

Absorption. — Water,  alcohol,  perhaps  to  some  ex- 
tent sugar,  may  be  taken  up  directly  by  the  capillaries 
of  the  stomach  into  the  blood.  But  absorption  is  the 
special  function  of  the  intestines.     The  chyme  is  there- 


NUTRITION    PROPER. 


315 


fore  passed  on  into  the  intestines  to  undergo  the  third 
stage  of  food  preparation.  There  we  leave  it  for  the 
present  while  we  take  up  the 

Comparative  Physiology  of  the  Stomach. — The 
chemical  process  of  digestion  is  the  same,  and  the  appa- 
ratus nearly  the  same  in  all  vertebrates.  There  are 
only  two  modifications  sufficiently  important  to  arrest 
our  attention — viz.,  that  of  ruminant  mammals  and  that 
of  granivorous  birds. 

Ruminants. — The  stomach  of  ruminants  (Fig.  200) 
is  very  complex,  and  the  whole  digestive  process  very 
elaborate.     The  stomach   consists  of  four  parts — viz. : 


Fig.  200. — Stomach  of  a  sheep,  partly  cut  open  so  as  to  show  the  interior: 
oe,  oesophagus  ;  v,  valvular  opening  from  the  oesophagus  to  the  omasum. 

(i)  the  rumefi  or  paunch;  (2)  the  reticulum  or  honey- 
comb ;  (3)  the  psalferiutn  or  omasum  (psalter,  or  book, 
or  manyplies) ;  and  (4)  the  abomasutn  or  rennet.  The 
rumen  or  paunch  (i)  is  of  immense  size,  and  its  func- 
tion is  to  store  the  half-chewed  food  and  soften  it  by 
maceration.  It  is  thick  and  muscular,  and  constitutes 
what  is  called  the  tripe.  The  reticulum  (2)  may  be  re- 
garded as  an  appendage  of  the  paunch,  and  its  function 
is  probably  to  prepare  a  macerating  liquid  and  perhaps 
also  to  make  up  the  cud-balls.     It  is  full  of  deep  pits, 


3i6   PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 

like  a  honeycomb.  The  psalter-  or  manyplies  (3),  as  its 
name  indicates,  has  its  mucous  membrane  thrown  into 
many  wide  and  thin  folds,  like  the  leaves  of  a  book.  It 
is  probable  that  the  digestion  of  starch  (saccharization) 
takes  place  here,  and  is  all  the  more  complete  because 
the  acid  gastric  juice  is  not  secreted  until  we  reach  the 
last  compartment.  Finally,  the  abomasum  or  rennet  (4) 
is  the  true  and  final  digestive  stomach,  where  pepsin  is 
formed  and  peptonization  takes  place. 

We  see,  then,  that  several  functions  which  are  com- 
bined in  the  same  organ  in  most  animals  are  here  dif- 
ferentiated with  more  perfect  results.  The  reason  of 
this  is  found  in  the  nature  of  the  food  and  the  habits  of 
the  animals.  The  food  consists  of  grasses  and  herbage, 
in  which  the  amount  of  nutritious  matter  is  so  small  that 
they  must  take  a  very  large  quantity,  and  therefore  the 
stomach  must  be  correspondingly  large.  But  again,  on 
account  of  the  small  percentage  of  nutritious  matter, 
the  food  must  be  very  thoroughly  triturated  and  the  di- 
gestive process  very  complete  so  as  to  extract  it  all. 
We  have  already  seen  how  well  their  teeth  are  adapted 
to  thorough  trituration;  we  see  also  how  the  stomach  is 
adapted  to  perfect  digestion.  But  perfect  trituration  of 
so  large  an  amount  of  food  would  take  much  time,  and 
these  animals  are  timid  and  preyed  upon  by  carnivores. 
Therefore  they  are  compelled  to  take  their  food  rapidly, 
imperfectly  chewing  and  hastily  packing  it  away  in  the 
paunch,  where  it  is  soaked  and  softened.  Then  at  their 
leisure  they  lie  in  some  concealed  place  and  naiiinate,  or 
chew  the  cud.  Every  one  must  have  observed  the  pro- 
cess in  domestic  animals,  and  perhaps  envied  their  pla- 
cidity of  mind.  If  we  observe  closely  we  see  the  chew- 
ing stop  a  while;  the  bolus  goes  down  the  gullet  by 
peristalsis,  then  another  comes  up  by  antiperistalsis, 
and  the  chewing  recommences  and  continues  until  the 


NUTRITION    PROPER. 


317 


cud  is  reduced  to  a  fine,  smooth  paste,  and  again  swal- 
lowed. 

Now,  when  the  imperfectly  chewed  food  is  swallowed 
the  first  time  it  finds  a  broad  open  way  to  the  paunch  ; 
but  when,  after  perfect  chewing,  it  is  swallowed  the  sec- 
ond time  the  powerful  muscles  about  the  oesophageal 
orifice  of  the  stomach,  by  a  reflex  action  little  under- 
stood, contract  in  such  wise  as  to  bring  the  orifice  of  the 
oesophagus  directly  into  contact  with  the  opening  into  the 
manyplies,  and  the  food  passes  into  this  compartment. 

Evolution  of  Ruminant  Stomach. — We  have  al- 
ready seen  that  the  teeth  of  ruminants  were  only  gradu- 
ally developed  in  geologic  times  from  a  simple  tubercu- 
lated  structure  into  the  complex  grinders  which  we  now 
find.  The  same  is  true  of  the 
complex  stomach,  but  the  evi- 
dence is  less  complete,  because 
the  stomachs  of  extinct  animals 
are  not  preserved.     Nevertheless,  \  P 

some  stages  are  still  found  in  ex- 
isting animals.     In  all  mammals.     Fig.   201.— Stomach  of  a 

,  .  ...        , . ,  rat :   r,  cardiac ;   p,    py- 

even  m  man,  there  is  a  slight  dif-         lork  portion, 
ference  of  function  in  the  cardiac 

and  pyloric  ends  of  the  stomach.  In  many,  as  the  horse, 
there  is  a  strong  line  of  demarcation  between  them.  In 
others,  as  rodents  (Fig.  201),  there  is  a  strong  hourglass 
contraction  between;  but  nowhere  is  the  differentiation 
so  marked  as  in  ruminants. 

Granivorous  Birds. — The  food  of  grain-eating 
birds  is  hard  and  requires  thorough  trituration,  yet  birds 
have  no  teeth,  and  therefore  mastication  and  insaliva- 
tion  can  not  take  place  in  the  mouth.  Their  food  is 
swallowed  whole  and  insalivated  in  the  crop,  and  masti- 
cated in  the  gizzard.  Fig.  202  gives  the  whole  appara- 
tus.    It  consists  of  three  parts — viz.,  (i)  the  crop  {inglu- 


3i8    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


vies),  a  reservoir  for  storing  and  softening  the  food  by  the 
saliva;  (2)  the, prove ntriculus,  v{\\\ch.  furnishes  the  peptic 
juice;  and  (3)  the  gizzard,  or  ventricidus,  or  gigerium, 
which  triturates  the  food  and  mixes  it  thoroughly  with 
saliva  and  gastric  juice  furnished  in  the  parts  above.     The 

gizzard  consists  of  two  very 
powerful  muscles,  provided 


Fig.  202. — Digestive  apparatus  of  a  Fig.   ■2o\. — Tiie  gizzard  of  a  goose 

granivorous  bird  :  cr,  crop ;   pv,  cut    open  :    tp,    triturating    pad. 

proventriculus ;  g,  gizzard;  /,  in-  (After  Owen.) 
testine. 

each  with  a  cushion  or  pad,  which  rub  against  one  an- 
other (Fig.  203).  The  cavity  is  small  and  lined  with  a 
hard,  almost  horny  skin.  Gravel  is  taken  as  grinders 
to  this  mill,  and  renewed  as  required.  The  whole  pro- 
cess is  briefly  as  follows:  The  food  is  first  stored  and 
insalivated  and  softened  in  the  crop.  The  crop  acts  as 
a  hopper  to  the  gizzard  mill,  dropping  little  by  little  as 
required.  The  digestive  juice  secreted  by  the  proven- 
triculus is  also  added  little  by  little  as  required.  The 
triturated  and  digested  material  tiiially  passes  on  to  the 
intestines. 

Evolution  of  this  Apparatus. — This  elaborate  ap- 
paratus is  most  perfect  in  grain-eating  birds  ;  but  all 
grades  approaching  it  may  be  found  in  birds  from  the 
simple  thin  sac  of  the  flesh-eating  to  the  powerful  mill 
of  the  grain-eating, 


NUTRITION    PROPER. 


319 


SECTION    IV. 
Intestinal  Digestion  :  Chylification  ;  Emiilsification. 

Intestines ;  Form,  Structure,  and  Relations  to 
the  Abdominal  Cavity.— I'he  intestines  consist  of  a 
long,  slender  tube — in  man  about  thirty-five  feet  long 
and  one  inch  to  two  inches  in  diameter.  It  is  divided 
into  two  very  distinct  parts — viz.,  the  small  and  large 
intestines.  These  differ  in  size,  the  small  being  about 
one  inch,  the  large  two  inches  in  diameter.  They  differ 
also  in  appearance,  the  first  being  smooth,  cylindrical, 
the  second  puckered  and  sacculated  by  a  strong  mus- 


FiG.  204. — The  junction  of  the  small  and  large  intestines  :  si,  small  intes- 
tines ;  //,  large  mtestines  ;  cae,  caecum  ;  vap,  vermiform  appendage. 

cular  band  (Fig.  204).  The  true  process  of  digestion  is 
substantially  completed  in  the  former.  The  function  of 
the  latter  is  not  well  understood,  but  both  the  charac- 
teristic color  and  odor  of  excrements  are  taken  on  here. 
Also  the  peculiar  shape  of  the  balls  m  the  horse  or  pel- 
lets of  sheep  and  goats  are  given  here. 

The   two   parts  do   not  grade  continuously  the  one 
into  the  other.     On  the  contrary,  the  small  open  into  the 


320   PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

large  on  one  side  a  little  way  from  the  end  by  a  valve 
(ileocsecal  valve;  Fig.  204).  Each  of  these  two  parts  are 
again  subdivided  into  three,  as  shown  in  the  schedule: 

i  Duodenum.  t  Caecum. 

Jejunum.  Large -s  Colon. 

Ileum.  '  Rectum. 

The  duodenum  is  the  part  next  the  stomach,  seen  in 
Fig.  198,  into  which  are  poured  the  digestive  juices 
from  the  liver  and  pancreas.  It  is  the  largest  and 
shortest  part  of  the  small  intestines,  being  about  ten 
inches  long  and  two  inches  in  diameter.  The  jejunum 
and  ileum  grade  completely  into  one  another  both  in 
structure  and  function.  Of  the  large  intestines,  the 
caecum  is  the  somewhat  enlarged  blind  extremity  into 
the  side  of  which  the  ileum  opens  (Fig.  204)  by  the  ileo- 
caecal  valve.  Attached  to  the  blind  extremity  there  is 
a  curious,  wormlike  appendage,  which  by  inflammation 
gives  rise  to  the  grave  disease  (appendicitis)  which  is 
now  attracting  so  much  attention.  The  rectum  is  the 
last  or  lower  part,  about  six  inches  long,  and  cylindrical 
in  form,  opening  through  the  anus.  The  colon  is  the 
sacculated  part  between  the  caecum  on  the  one  hand 
and  the  rectum  on  the  other,  and  constitutes  the  prin- 
cipal part  of  the  large  intestines.  The  caecum  is  in  the 
lower  right-hand  part  of  the  abdomen.  The  colon  runs 
from  it  upward  on  the  right  side  to  the  region  of  the 
stomach,  then  across  to  the  left,  and  then  down  the  left 
side  to  the  rectum. 

Relations  to  the  Abdominal  Walls.— So  long 
and  so  slender  a  tube  must  be  so  held  in  place  that  it  be 
not  tangled  ;  and  also  it  must  be  in  easy  reach  of  the 
great  vessels  which  supply  it  with  blood,  and  which  take 
away  the  digested  food.  This  is  done  by  a  thin,  trans- 
parent membrane  which  is  attached  by  one  edge  to  the 


NUTRITION    PROPER. 


321 


backbone,  and  by  the  other  to  the  whole  length  of  the 
intestine,  following  all  its  complex  windings.  This  is 
called  the  mesentery  (Fig.  205).     From  this  arrangement 


Fig.  205. — Small  intestines  attached  to  the  mesentery. 


it  follows  that  the  intestines  are  nowhere  more  than  six 
inches  away  from  the  great  vessels  lying  along  the 
backbone. 

PeritoncButn. — It  may  seem  paradoxical,  but  is  never- 
theless in  some  sense  true,  that  the  intestines,  and  in- 
deed all  the  abdominal  viscera,  are  outside  the  abdomi- 
nal cavity.  The  whole  abdominal  cavity  {vc,  Fig.  206) 
is  lined  with  smooth,  shining,  serous  membrane  called 
the  peritonecum  {per).  This,  on  reaching  the  backbone, 
is  reflected  forward  as  a  double  membrane,  the  mesen- 
tery, and  then  over  the  intestine  as  its  investing  coat. 
If  this  serous  membrane  could  be  dissected  off  com- 
pletely it  would  form  a  complete  sac  without  opening. 


322    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

What  we  mean  by  saying  that  the  intestines  are  outside 
of  the  peritoneal  cavity  is  shown  in  Fig.  206. 


Fig.  206. — Diag;ram  showing  the  relation  of  the  intestines  to  the  abdominal 
cavity  :  z',  intestine  ;  vc,  visceral  cavity  ;  per,  peritonaeum  ;  nt,  muscular 
coat ;  £■/,  epithelial  coat ;  sp,  spinal  column  ;  ao,  aorta  ;  spc,  spinal  cord. 
(After  Wiedersheim.  i 

Coats  of  the  Intestines. — Like  the  stomach,  the 
intestines  have  three  coats — the  outer,  peritoneal ;  the 
middle,  mtiscular;  and  the  inner,  epithelial.  In  other 
words,  it  is  a  hollow,  muscular  tube,  invested  with  the 
peritoneal  coat  and  lined  with  epithelial  or  mucous  mem- 
brane. The  peritoneal  coat  gives  smoothness,  the  mus- 
cular coat  does  the  mechanical  work,  and  the  epithelial 
the  chemical  work  of  digestion. 

Mechanical  Work. — The  muscular  coat  consists 
mainly  of  two  sheets  of  parallel  fibers.  In  the  outer 
one  the  fibers  run  lengthwise,  in  the  inner  ringwise. 
Under  the  contraction  of  these  the  intestines  may  be 
seen  to  squirm  wormlike  from  side  to  side,  and  light 
waves  of  contraction  may  be  seen  running  always  in 
one  direction — downward.  This  is  the  peristaltic  action 
of  the  intestines.     The  waves  must  be   light,  otherwise 


NUTRITION    PROPER. 


323 


the  food  would  be  too  much  hurried  along  its  way. 
Slowly,  therefore,  the  digested  food  is  urged  along.  In 
the  meantime  the  absorbents  are  taking  up  the  liquid 
parts.  The  percentage  of  liquids  decreases,  until  finally 
only  solid,  indigestible  parts  remain.  This  is  finally 
pushed  through  the  ileocoa^al  valve  (Fig.  204)  into  the 
large  intestines,  and  the  digestion  is  substantially  done. 
There  are  changes  of  an  obscure  kind  which  go  on  there, 
but  these  are  too  little  known  to  detain  us. 

Chemical  Work. — The  digestive  juices  of  intesti- 
nal digestion  are  three — viz.,  the  bile,  the  pancreatic 
juice,  and  the  intestinal  secretion.  The  general  effect 
of  these  is  to  produce  a  milky  liquid  called  chyle. 

[ci)  The  bile  is  secreted  by  the  liver,  and  during  diges- 
tion is  poured  out  in  large  quantities  into  the  duodenum 
a  little  way  below  the  stomach  through  a  common  duct 
made  up  of  the  union  of  the  bile  duct  with  the  pancre- 
atic duct  (Fig.  198).  The  digestive  effect  of  the  bile  is 
manifold,  i.  It  will  be  remembered  that  the  chyme  is 
acid,  and  that  acidity  is  unfavorable  to  the  sacchariza- 
tion  of  starch.  Thus  it  happens  that  some  starch  may 
escape  solution  in  the  stomach.  Now,  the  bile  is  alka- 
line, and  therefore  neutralizes  the  acidity  of  the  chyme, 
and  thus  revives  the  activity  of  the  ptyalin  on  the 
starches.  2.  It  will  be  remembered,  again,  that  of  the 
three  kinds  of  food  we  have  had  digestive  juices  for 
two,  viz.,  ptyalin  for  starches  and  pepsin  for  albumi- 
noids; but  the  fats  have  not  yet  been  touched.  Now, 
the  alkalinity  of  bile  partially  saponifies  the  fats,  and 
thus  prepares  them  for  emulsification  by  the  pancre- 
atic and  intestinal  juices.  3.  It  is  found,  too,  that  in 
order  to  be  absorbed  easily  a  liquid  must  be  either 
neutral  or  a  little  alkaline.  Thus  the  bile,  by  neutral- 
izing the  acidity  of  the  chyme,  prepares  it  for  easy 
absorption. 


324 


PHYSIOLOGY    AND    MORPHOLOGY    OF  ANIMALS. 


[b)  The  pancreas  [siveetbread)  lies  just  below  the 
stomach,  and  its  excretory  duct  unites  with  the  bile  duct 
to  open  by  a  common  duct  into  the  duodenum.  Its 
structure  is  similar  to  that  already  described  in  the  sali- 
vary gland.  Its  secretion  is  perhaps  the  most  impor- 
tant of  all  the  digestive  juices.  It  performs  the  func- 
tion of  all  previously  mentioned,  and  supplements  them 
all.  It  saccharizes  starch,  like  the  saliva.  It  peptonizes 
albuminoids,  like  the  gastric  juice.  It  is  slightly  alka- 
line, like  the  bile,  and  it  is  a  powerful  emulsifier  of  fat, 
like  the  intestinal  mucus.  These  properties  are  the 
result  of  several  ferments,  among  which  may  be  men- 
tioned paucreatin  or  aiiiylopsin  and  trypsin,  the  former  a 
solvent  of  starch  and  the  latter  of  albuminoids.  What- 
ever of  albuminoids  or  amyloids  escape  digestion  in  the 
stomach  are  dissolved  here.*  It  also  forms  still  another 
ferment  which  splits  fats  into  glycerin  and  fatty  acids. f 
In  addition  to  all  these  the  pancreas  has  still  other 
functions,  which  will  be  discussed  later.  Suffice  it  to 
say  now  that  it  apparently  delivers  a  peculiar  ferment 
directly  to  the  blood. 

{c)  The  intestinal  secretion  probably  has  other  diges- 
tive properties  little  known,  but  certainly  by  its  slimy 
viscidity  it  is  very  efficient  in  the  emulsification  of  fats. 

Emulsion. — We  have  said  that  chyle  is  a  milky 
liquid.  Its  whiteness  is  wholly  due  to  emulsified  fats. 
We  explain  this  as  follows:  Ice  is  transparent,  but  break 
it  up  into  fine  particles  like  snow  and  it  is  intensely 
white.  Glass  is  transparent,  but  grind  it  to  fine  powder 
and  it  is  white.  Water  is  transparent,  but  spray  and 
foam  are  white.     And  so  in  all  cases  of  whiteness.     Oils 

*  Recently  shown  (Archives  des  Sciences,  iv,  490,  1897)  that  the 
spleen  furnishes  a  product  necessary  to  the  formation  of  trypsin. 
f  Am.  Nat.  xxxi,  1040,  1897. 


NUTRITION    PROPER. 


325 


and  fats  are  transparent,  but  broken  up  into  microscopic 
globules  they  are  also  white.  Now,  what  is  called  an 
emulsion  consists  of  millions  of  microscopic  globules  of 
oil  swimming  in  a  transparent  liquid,  but,  in  order  that 
the  globules  do  not  run  together  and  unite,  the  liquid 
must  be  viscid. 

Experiment. — Take  a  small  bottle,  fill  it  partly  with 
a  viscid  liquid  like  gum  water  or  mucus,  and  then  pour 
in  a  little  oil  of  some  kind,  as  turpentine  or  sweet  oil. 
We  see  them  as  two  equally  transparent  layers  one  atop 
the  other.  Now,  putting  the  thumb  on  the  mouth  of 
the  bottle,  shake  it  violently.  The  result  is  an  intensely 
white  fluid,  which,  examined  by  microscope,  shows  noth- 
ing but  transparent  globules  floating  in  a  transparent 
liquid.  This  is  an  emulsion.  Milk  is  also  an  emulsion, 
and  its  whiteness  is  due  to  the  same  cause.  Chyle  is  an 
emulsion,  and  its  whiteness  is  due  to  the  presence  of 
microscopic  fat  globules.  If  there  be  no  fat  in  the  food, 
the  chyle  will  be  transparent.  In  the  process  of  diges- 
tion the  fats  and  oils  are  broken  up  by  the  constant 
pressing  and  kneading  ac- 
tion of  the  intestines  in  the 
presence  of  a  viscid  liquid. 

Absorption. — The 
three  kinds  of  food  are 
now  all  become  absorb- 
able. The  starches  have 
become  sugars,  the  albu- 
minoids peptones,  and  the 
fats  emulsions.  The  food 
is  now  ready  for  absorp- 
tion. But  for  rapid  ab- 
sorption we  must  have  a 
large  surface.  This  con- 
dition   is   supplied    partly 


Fig.  207. — Interior  of  the  intestines, 
showing  the  valvula;  conniventes. 


326 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANLMALS. 


Fig.  208. — Two  of  the  villi,  greatly 
enlarged  ;  one  showing;  blood  cap- 
illaries, the  other,  the  lacteals : 
ep,  epithelium. 


by  the  great  length  of  the  intestines,  still  more  by  the 
many  folds  of  the  mucous  membrane  {T.'alviilce  conniventes) 
(Fig.  207),  but  most  of  all  by  the  villi.  These  micro- 
scopic hairlike  projections 
cover  the  surface  as  thick- 
ly as  the  pile  of  velvet  or 
the  tufts  of  Brussels  car- 
pet. They  give  a  velvety 
feel  to  the  mucous  mem- 
brane (Fig.  208).  Each 
villus  contains  two  kinds 
of  absorbent  vessels — viz., 
capillary  blood  vessels  and 
lacteals.  These,  though 
separated  in  the  figure, 
are  both  of  them  in  each 
villus. 
Two  Modes  of  Absorption. — The  food  is  ab- 
sorbed in  two  ways — viz.,  by  blood  capillaries  and  by  lac- 
teals. The  blood  capillaries  contain  a  circulati?ig  current^ 
and  therefore  take  up  the  food  and  carry  it  along  with 
the  blood.  The  lacteals,  on  the  contrary,  are  purely 
absorbent,  and  they  end  in  blind,  fingerlike  extremities, 
and  therefore  suck  up  the  liquid  food.  Whatever  of  food 
is  absorbed  by  the  stomach  is  wholly  by  blood  capil- 
laries. But  the  intestines  are  specially  organized  for 
absorption  in  both  ways.  The  food  is  divided  between 
these  two  modes.  The  sugars  are  absorbed  mainly  by 
the  capillaries,  the  fats  by  the  lacteals,  while  the  pep- 
tones are  divided  between  them. 

Course  of  Each  to  the  General  Circulation. — 
That  taken  up  by  the  capillaries  is  carried  to  the  por- 
tal vein  ;  thence  it  passes  through  the  liver,  and  by  the 
hepatic  vein  it  reaches  the  vena  cava  ascendens  and  by 
it  is  carried  to  the  heart,  and  is  thence  distributed  every- 


NUTRITION    PROPER. 


327 


where.  That  taken  up  by  the  lacleals  is  carried  by  the 
lacteal  or  lymphatic  vessels  of  the  mesentery  through 
the  mesenteric  glands  into  the  receptaculum  chyli,  and 
thence  up  to  the  thoracic  duct,  the  lymphatic  trunk 
lying  along  the  backbone  on  the  left  side  (Fig.  209). 
This  duct  rises  to  the  collar  bone,  then  turns  downward 
and  empties  into  the  angle  formed  by  the  junction  of 


01/  c 


\J|lMi)t::ii!J 


Fig.  209. — A  seg^nent  of  the  intestines  ('«'\  shownng;  the  lacteal  vessels  pass- 
ing along  the  mesentery  im  \,  through  the  mesenteric  glands  \mgl),  and 
into  the  thoracic  duct  {thd)  :  avc,  ciscending  vena  cava. 

the  subclavian  vein  from  the  left  arm  and  the  jugular 
coming  from  the  head,  and  thence  it  passes  by  the  vena 
cava  descendens  directly  to  the  heart,  to  be  distributed 
to  the  whole  body.  This  will  be  explained  more  fully 
hereafter. 

Sanguificatioti. 

In   the  course  of  each  of  these  to  the  general  circu- 
lation there  are  certain  changes  which  assimilate   it  to 


328    PHYSIOLOGY   AND    MORPHOLOGY    OF  ANIMALS. 

the  character  of  the  blood,  (i)  We  have  said  that  that 
which  is  taken  up  by  the  capillaries  passes  into  the  vena 
porta.  Now  this  is  a  quite  unique  and  exceptional  vein. 
All  other  veins  ramify  at  one  end  only;  this  at  both 
ends.  All  other  veins,  after  receiving  their  supply  by 
the  capillaries  from  the  tissues,  empty  by  an  open  mouth 
into  the  general  circulation.  This  one  receives  its  sup- 
ply from  the  capillaries  of  the  intestines,  the  stomach, 
and  the  spleen,  and  then,  instead  of  emptying  mto  the 
vena  cava,  close  at  hand,  goes  to  the  liver,  to  be  again 
distributed  hy  inverse  capillary  ramification  through  that 
organ,  and  gathered  a  second  time  by  the  capillaries  of 
the  hepatic  vein,  and  so  delivered  to  the  general  circu- 
lation. Thus  there  is  a  mesenteric  artery,  but  no  corre- 
sponding mesenteric  vein.  There  is  a  splenic  artery,  but 
no  splenic  vein.  The  vena  porta  takes  the  blood  of  both 
these,  and  carries  it  to  the  liver,  to  be  again  distributed 
there.  This  is  shown  in  the  diagram  (Fig.  210).  Now, 
during  the  passage  of  the  digested  food  through  the 
liver  it  undergoes  an  important  change,  preparing  it  for 
immediate  use  in  the  generation  of  force  and  heat.  In 
other  words,  it  is  sanguified — i.  e.,  made  into  material 
suitable  for  circulation  in  the  blood.  The  nature  of  this 
important  change  we  will  explain  more  fully  when  we 
come  to  speak  of  the  functions  of  the  liver  (page  446). 

That  part  of  the  dissolved  food  which  is  taken  up  by 
the  lacteals,  as  we  have  seen,  is  carried  through  the  mes- 
enteric glands  before  reaching  the  thoracic  duct.  In 
these  glands  an  important  change  takes  place.  After 
passing  through  these  it  is  no  longer  mere  dissolved 
food — i.  e.,  peptones  and  fats — but  apparently  a  living 
fluid  like  the  blood,  although  not  yet  red.  Before  pass- 
ing through  the  glands  it  is  noncoagulable ;  after  pass- 
ing it  is  coagulable,  like  the  blood.  Before  passing  it 
contains  no  globules  except  fat  globules.     After  passing 


NUTRITION    PROPER. 


329 


it  contains  also  living  nucleated  cells  like  the  white  cor- 
puscles of  the  blood,  contributed  to  it  by  the  glands. 
In  a  word,  before  passing,  it  is  chyle ;  after  passing,  it  is 


Fig.  210. — Diagram  showing  the  distribution  of  the  blood  through  the  liver. 
The  arrows  sliow  the  direction  of  the  current. 

at   least   partly   converted   into   blood.     The   change  is 
completed  in  the  blood  itself. 

The  last  stage — i.  e.,  the  actual  assimilation  into  tis- 
sue— will  be  spoken  of  in  connection  with  the  circulation. 

MODIFICATION    OF     THE     PROCESS     OF     INTESTINAL 
DIGESTION     IN    VERTEBRATES. 

There  is  little  to  be  said  on  this  subject,  because  the 
modification  of  the  process,  as  already  given,  is  unimpor- 
tant. The  whole  process  of  digestion — mouth  digestion, 
stomach  digestion,  and  intestinal  digestion — is  simpler  in 
carnivores  and  more  elaborate  in   herbivores.     Man  in 


330 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


this  regard,  as  in  so  many  others,  may  be  regarded  as  a 
generalized  mean.  The  intestinal  canal  is  shorter  in  car- 
nivores and  much  longer  in  herbivores  than  m  man.  It 
may  be  well,  however,  to  mention  one  or  two  interesting 
points.  The  csecum,  which  in  man  is  but  a  slight  en- 
largement of  the  blind  end  of  the  large  intestine  (see 
Fig.  204),  is  greatly  enlarged  in  the  horse,  and  in  the 


Fig.  211. — The  alimentary  canal  of  a  rat  :  sf,  stomach  ;  ca,  cacum. 
(From  Owen. ) 

rat  is  indeed  a  sort  of  second  stomach  as  big  as  the  true 
stomach  (Fig.  211).  The  most  interesting  thing  in  this 
connection  is  the  origin  of  the  strange  wormlike  appen- 
dix. It  is  probably  the  remnant  of  the  shriveling  of  the 
extreme  end  of  a  much  longer  caecum.  It  is  an  example 
of  a  useless  remnant  of  a  once  useful  organ. 


NUTRITION    PROPER. 


331 


One  more  example:  In  fishes  the  whole  digestive 
process  is  very  simple,  and  the  intestines  are  correspond- 
ingly short.  But  in  the  shark,  although  externally  the 
intestine  is  almost  a  straight  tube  running  through  the 
body,  yet  iutcmally  its  surface  of  absorption  is  made  very 
large  by  means  of  a  curious  spiral  valve.  This  gives 
the  peculiar  spiral  marking  on  the  dung  of  these  and 
of  some  other  lower  vertebrates,  both  living  and  extinct, 
for  fossil  dung  of  sharks  and  reptiles  is  not  uncommon. 

SECTION  V. 
Digestive  Systeiti  in  Invertebrates. 

General  Remarks. — i.  As  already  explained,  we 
shall  pursue  a  different  plan  here.  Instead  of  following 
each  stage  of  digestion  through  the  different  classes,  we 
shall  run  through  the  whole  digestive  process  in  each 
class  as  taken  up. 

2.  The  complexity,  and  especially  the  diversity, 
among  invertebrates  is  so  great  that  if  we  took  them  up 
with  anything  like  the  fullness  that  we  have  taken  up 
vertebrates,  our  object — viz.,  to  make  a  small  volume, 
giving  only  an  outline  of  the  most  interesting  points — 
would  be  defeated.  We  will,  therefore,  only  give  a  few 
very  striking  examples  from  different  departments. 

ARTHROPODS. 

Insects  :  Mouth  Parts. —  Insects  are  wonderfully 
specialized  animals.  This  is  especially  true  of  their  mouth 
parts.  From  this  point  of  view  insects  may  be  divided 
into  two  groups — viz.,  the  biting  and  the  sucking  in- 
sects, or  the  7uaudibulate  and  the  haustellate.  The  former 
include  the  orthopters  (grasshoppers),  the  neuropters 
(dragon  flies),  and  the  coleopters  (beetles).  The  latter 
include  the  lepidopters  (butterflies  and  moths),  the  dip- 


332 


o   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


ters  (flies),  and  the  hymenopters  (bees  and  ants),  al- 
though these  last  are  intermediate.  Of  these  two  kinds 
of  mouth  parts  the  former  is  undoubtedly  the  original 
and  normal,  of  which  the  latter  must  be  regarded  as  an 
extreme  modification,  for  the  mandibulate  form  alone 
is  found  in  the  larval  state  (e.  g.,  caterpillars)  and  in 
early  geological  times. 

Normal  Mouth  Parts. — Taking,  then,  the  orthop- 
ter,  or  grasshopper,  or  else  a  beetle,  as  example,  there 

are  six  movable  mouth 
parts;  but  as  four  of 
these  are  in  pairs,  we 
may  say  there  are  four 
kinds.  These  are  the 
labruni  (so-called  upper 
lip),  the  two  mandibles., 
the  two  m  axil  Ice,  with 
their  jointed  append- 
ages, and  the  labium 
(lower  lip),  with  its 
jointed  appendages. 
Sometimes  there  is  an 
elongation  of  this  last 
called  the  tongue. 
These  are  seen  in  place 
in  the  beetle  (Fig.  212), 
and  the  same  parts  in  a 
grasshopper  separated 
to  display  their  forms 
and  position  in  Fig.  213.  Looking  directly  at  the  mouth 
of  the  grasshopper  or  beetle,  their  relative  position  and 
direction  of  motion  is  shown  in  the  diagram  (Fig.  214). 
The  food  is  gathered  by  the  maxillary  and  labial  palpi, 
is  placed  between  the  jaws  (mandibles  and  maxillae), 
pressed  together  between  the  upper  and  lower  lips,  and 


Fig.  212. — Head  of  a  beetle  seen  from 
behind  :  lb,  labium  ;  Ip,  labial  palpi ; 
m  VI,  mandibles  ;  mx,  maxillae  ;  mp, 
maxillary  palpi ;  /br,  labrum. 


NUTRITION    PROl'ER. 


333 


divided  and  masticated  by  the  jaws  working  toward  one 
another  laterally  (not  vertically,  as  in  vertebrates). 


mxp\- 


.'y  nwcp 


Fig.  213. — The  mouth  parts  of  a  grasshopper  enlarged  :  Ibr,  labrum  ;  m  m, 
mandibles;  /d,  labium;  //>,  labial  palpi;  /.tongue;  tnx rnx,  maxillae; 
mxp  tnxp,  maxillary  palpi. 


Serial  Homology  of  these  Parts.— As  already 
said  (page  273),  the  insect  head  consists  of  four  segments 
with  their  paired  appendages.  The  labrum  is  an  exten- 
sion downward  of  the  first  segment,  and  the  antenniT  are 
the  jointed  appendages;  the  mandibles  are  the  paired 
appendages  of  the  second  segment;  the  maxilhx,  the 
paired  appendages  of  the  third  segment ;  while  the  la- 
bium and  its  paired  appendages  belong  to  the  fourth 
segment  (Fig.  215). 


334 


PHYSIOLOGY    AND    MORPHOLOGY   OP^   ANIMALS. 


Special  Homolog'y. — In  suctorial  insects  the  mouth 
parts  are  an  extreme  adaptive  modification — so  extreme 
in  some  cases  that  their  homology  with  those  of  masti- 


JliT 


mx- 


ib 


Fig.  214.  —  Diagram  showing;  the 
relative  positions  and  direction  of 
motion  in  mastication.  The  arrows 
show  the  direction  of  motion. 


Fig.  215. — Side  view  of  the  head  of 
a  gfrasshopper  :  e,  eye  ;  other  let- 
ters are  tlie  same  as  in  Figs.  212, 
213,  and  214. 


Fig.  216. — Head  of  a  butterfly  seen 
from  in  front.  The  letters  show 
the  same  parts  as  in  previous  fig- 
ures. 


catory  insects   is   somewhat   doubtful.      We   give  what 
seems  the  most  probable  view  in  two  extreme  forms: 

Butterfly. — In  this  case  the  labrum,  Ibr,  and  the  man- 
dibles, m  m  (Fig.  216),  are  rudimentary  and  useless. 
The  maxillae,  vix  7!ix,  are  enormously  elongated  into 
muscular  hollow  semicylinders,  which,  when  put  together, 
form  a  long,  flexible,  hollow  sucking  tube.  This  is  usu- 
ally carried  coiled  up  like  a  watch  spring,  but  is  uncoiled 
and  straightened  when  used  as  a  proboscis  for  sounding 
the  nectar  tubes  of  flowers. 


NUTRITION    PROPER. 


33: 


Bees  are  a  good  example  of  intermediate  form  (Fig. 
217).  In  these  the  labrum  and  mandibles  are  as  usual, 
but  the  maxillae  and  labium,  with  their  appendages,  are 


Fig.  217.— Head  of  a  bee  seen  from  Fig.  218. — A.  enlarged  view  of  a  bee's 

behind.     The  labrum  is  not  seen.  tong:ue  ;    r,  inner  tube  ;   B,  section 

The  other  parts  are  lettered  as  be-  more  enlarged,  showing  its  struc- 

fore  ;  t,  tongue.  ture. 

greatly  elongated  to  form  a  sucking  organ.  This  is 
especially  true  of  that  projection  of  the  labium  called 
the  tongue,  /,  which  is  modified  into  an  apparatus  of 
marvelously  complex  structure.  This  is  shown  in  Fig. 
218,  in  which  A  shows  the  outer  tube  with  the  inner  tube 
23 


336   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIiMALS. 


pulled  out;  B,  a  section  of  the  whole  when  the  inner 
tube  is  in  place.  The  bee,  therefore,  both  bites  and 
sucks.  If  the  nectary  tube  of  a  flower  is  too  long  to 
reach  the  nectar  with  the  sucker,  he  bites  the  nectary 
and  reaches  the  honey  more  directly. 

Digestive  Apparatus. — Fig.  219  shows  the  whole 
digestive  apparatus  of  a  beetle.      In  Fig.  220  we  give 


-nit 


Fig.  219. — Dii^estive  apparatus  of  a  bee- 
tle :  m,  mandibles  ;  a,  antenna  ;  ce, 
oesophagus  ;  c,  crop  ;  prz',  proventricu- 
lus  ;  s/,  stomach  ;  dd,  biliary  ducts  ;  d, 
duodenum  ;  i,  intestines  ;  ;-,  rectum  ; 
ag-/,  anal  gland. 


Fig.  220. — Digestive  system 
of  a  Belostoma :  /t,  head ; 
ce,  oesophagus :  sg,  salivary 
gland  ;  sr,  salivarj'  recepta- 
cle ;  s^,  stomach  ;  mi,  Mal- 
pighian  tubes  ;  ccc,  caecum. 
(After  Packard.) 


NUTRITION    PROPER. 


337 


also  the  same  in  another  insect  [Belostoma)  to  show  the 
variations,  and  especially  to  show  the  salivary  glands, 
not  shown  in  the  previous  figure.  The  distinctive  func- 
tions of  these  several  parts  are  somewhat  doubtful. 
The  most  probable  view  is  given  in  the  legends. 

We  have  given  the  simplest  case  of  a  carnivorous  bee- 
tle. In  many  insects,  especially  the  herbivorous,  like  the 
grasshopper  (orthopter),  the  digestive  apparatus  is  much 
more  complex  and  the 
intestines  much  longer 
and  more  convoluted. 

It  is  well  to  remark 
that  what  we  have  called 
the  biliary  or  Malpighi- 
an  tubes  are  also,  appar- 
ently, uriniferous  tubes 
as  well.  These  two  func- 
tions are  not  yet  well 
differentiated.  The  sig- 
nificance of  this  will  ap- 
pear hereafter. 

Crustaceans: 
Mouth  Organs.  —  We 
have  already  seen  (page 
268)  that  these  are  all 
modified  appendages  of 
the  anterior  somites. 
Four  pairs,  called  max- 
illipeds,  are  food  gather- 
ers, and  two  pairs,  max- 
illae and  mandibles,  di- 
vide and  chew  the  food 
(see  Fig.  170). 

Stomach. — The  stomach  is  situated  in  the  anterior 
part    of     the    cephalothorax,    immediately    above    the 


Fig.  221. — Lobster  with  the  carapace 
taken  off :  st,  stomach  ;  dotted  tube, 
ii,  intestines;  Z,  liver;  H,  heart. 


338    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

mouth  and  connected  therewith  by  a  short  oesophagus. 
If  we  take  off  the  carapace  of  a  lobster  by  dividing  the 
attachments  with  the  sternum,  F'ig.  221  will  represent 
what  we  see.  Immediately  beneath  the  carapace  is  the 
heart,  H,  with  the  blood  vessels  running  fore  and  aft. 
We  have  nothing  to  do  with  these  now.  We  will  speak 
of  them  later.  In  front  and  beneath  the  heart  is  seen 
the  large  stomach,  st,  with  the  straight  intestine,  /,  run- 
ning backward  beneath  the  great  artery.  Behind  the 
stomach,  on  each  side  of  the  anterior  part  of  the  intes- 
tine, is  seen  the  large  liver,  Z,  composed  of  many  tubules 
opening  into  the  intestine.  The  stomach  is  very  mus- 
cular, and  if  opened  is  seen  to  contain  what  might  be 
called  sto?nach  teeth — i.  e.,  powerful  triturating  organs 
composed  of  chitin  and  armed  with  prickles  of  the  same. 

MOLLUSCA. 

We  take  one  example  from  each  class. 
Acephala. — Among  these  we  take  the  Mactra  (Fig. 
222).     In  all  acephala  the  food-taking  is  wholly  involun- 


Fig.  222. — Mactra  with  one  valve  removed,  showing  the  anterior  iatn)  and 
the  posterior  (prn)  shell  muscles,  and  the  foot  (/").    (From  Gegenbaur.) 

tary.  The  clam  buries  itself  in  mud  with  the  mouth 
downward  and  the  siphon  upward,  just  reaching  the 
water.  By  ciliary  action,  currents  are  created  which 
pass  down  one  tube  of  the  siphon,  through  the  gills, 
contributing  thus  to  respiration  ;  then  to  the  mouth, 
contributing    thus    to    alimentation;    then    back     again 


NUTRITION    PROPER. 


339 


through  the  gills  and  out  by  the  other  tube  of  the  siphon, 
carrying  with  it  refuse  or  excretions  of  all  kinds.  The 
most  remarkable  thing  about  the  digestive  apparatus  is 
the  enormous  liver  (nearly  the  whole  of  the  dark  part  of 


Fig.  223. — \'^ertical  longitudinal  section  of  Anodonta  :  pm,  a»i,  posterior- 
anterior  adductor  muscles  ;  m,  mouth  ;  st,  stomach ;  cce,  csecum  ;  Hy 
heart ;  /",  foot ;  G,  gills  ;  ma,  mantle. 

an  oyster  or  a  clam  is  liver),  through  which  the  long  and 
convoluted  intestine  winds,  receiving  in  its  course  the 
biliary  secretion  from  many  openings,  and  then,  strange 
to  say,  passing  through  the  heart  on  its  way  backward 
to  the  vent  at  the  si- 
phon. The  stomach 
and  the  winding  course 
of  the  intestine  is  shown 
in  Fig.  223. 

Gastropoda.— For 
an  example  of  these 
take  a  snail.  Gastro- 
pods are  much  more 
highly  organized  than 
the  acephala.  They 
have    a    distinct    head 


Fig.  224. — Section  through  snout  of  a  car- 
nivorous gastropod  showing  the  radu- 
la,  r,  in  place :  A',  lingual  cartilage ; 
tnp/i,  muscle  of  pharynx  ;  m,  mouth  ; 
£p,  oesophagus.     (After  Lang.) 


and  their  food-taking  is  voluntary.    The  mouth  is  armed 
with  transversely  ridged   chitinous   plates,  which   have 


340 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


been  called  jaws,  and  a  ribbon  of  chitin,  called  radula, 
thickly  set  with  sharp  teeth,  by  which  they  rasp  their 
vegetable  food.  In  carnivorous  conchs  the  radula  is 
used  for  boring  round  holes  through  the  shells  of  other 

species  for  the  purpose 
of  sucking  their  juices 
(Figs.  224  and  225).  The 
intestine,  as  in  acephala, 
winds  through  the  liver, 
and  in  shelled  forms  turns 
forward  to  discharge  at 
the  opening  of  the  shell. 
Here,  in  fact,  in  the  snail 
we  find  four  tubes  open- 
ing: (i)  Of  course,  the 
mouth  ;  (2)  the  intestinal 
opening  (anus) ;  (3)  the 
genital  opening  ; 
and  (4)  the  respir- 
atory opening. 

Cephalopoda. 
— For  this  class 
we  take  the  squid 
{Sepia).  Squids 
take  their  food 
by  means  of  the 
powerful  muscu- 
lar arms  surrounding  the  mouth  (Fig.  226)  and  divide 
it  with  their  powerful  parrotlike  beak  (Fig.  227),  which 
moves  up  and  down  like  the  jaws  of  vertebrates,  not 
laterally,  like  those  of  arthropods.  The  digestive  tube 
is  very  simple,  the  long  oesophagus  passing  into  the 
large  stomach  and  the  intestines  coming  thencefor- 
ward to  discharge  in  front,  so  that  the  debris  is  carried 
away  by  the  water  currents  of  respiration.     These  ani- 


FiG.  225. — Different  forms  of  the  teeth  of 
carnivorous  gastropods. 


NUTRITION    PROPER. 


341 


mals  are  the  most  highly  organized  among  mollusca. 
All  the  organs  contributive  to  digestion,  such  as  salivary 
glands,  liver,  and  perhaps  pancreas,  are  found. 


Fig.  226, — Diagram  of  digestive  system  of  a  squid  :  eg,  cephalic  ganglion  ; 
ceg,  oesophageal  ganglion  ;  ce,  oesophagus ;  st,  stomach  siphon ;  mg, 
mantle  ganglion  ;  vg,  visceral  ganglion  ;  H,  heart ;  G,  gills. 

ECHINODERMS. 

In  the  echinus,  or  sea  chestnut,  we  have  perhaps  the 
most  elaborate  jaw  structure  to  be  found  in  the  whole 
animal  kingdom — viz.,  the   so-called  "  lantern   of  Aris- 


FiG.  227. — Jaws  of  a  squid  : 
«/,  upper  jaw  ;  /,  lower  jaw. 


Fig.  228. — The  masticating  appa- 
ratus of  an  echinus. 


totle  "  (Fig.  228).  This  consists  of  five  three-sided  hol- 
low prismatic  pieces  surrounding  the  oesophagus  and 
fitted  together  so  as  to  make  a  somewhat  conical  whole. 
Through   each  prismatic  piece  there  runs,  as  through  a 


342 


PHYSIOLOGY    AND    MORPHOLO(;Y    OF   ANIMALS. 


sheath,  a  long,  slender,  slightly  curved  rod,  sharp  and 
enameled  at  the  point.  These  prismatic  pieces  with 
their  sharp  teeth  are  worked  by  powerful  muscles,  so 
as  to  move  radially  to  and  from  the  center.  The 
food  is  taken  by  curious  nipping  appendages  (pedi- 
cellarife)  and  handed  on  to  the  mouth  and  divided 
and  chewed  by  the  lantern  apparatus.  The  stomach  is 
immediately  above  the  lantern  ;  the  intestine  is  much 
convoluted,  and  emerges  by  opening  in  the  center  of  the 
radiated  upper  surface  of  the  animal. 

CCF.LENTERATES. 

Thus  far  we  have  found  intestinal  as  well  as  stom- 
achal digestion.  The  food  preparation  is  carried  to 
the  condition  of  chyle.  In  all  such  cases  there  is  a  blood 
system  distinct  from  the  digestive  system,  and  into 
which  the  chyle  is  absorbed,  to  become  afterward 
changed  into  blood.  In  all  animals  spoken  of  thus  far 
blood  and  digested  food  are  distinct  from  one  another. 
The  digested  food  is  absorbed  and  changed  into  blood 
(sanguified).  But  now  we  find  a  great  change  in  this  re- 
gard. In  the  coelenterates,  and  below,  there  is  no  longer 
any  intestine  as  distinct  from  the  stomach.  The  diges- 
tion goes  only  as  far  as  chyme-making.  There  is  no 
longer  any  distinction  between  the  digestive  system  and 
the  blood  system,  nor  between  digested  food  and  blood. 
The  digested  food  is  the  only  nutrient  fluid.  We  take 
as  an  excellent  example  of  coelenterates  the 

Medusa,  or  Jellyfish,  or  Sea-Blubber. — These 
beautiful,  almost  transparent,  saucerlike  or  bell-shaped 
animals  (Fig.  229),  with  their  long  trailing  tentacles  and 
graceful  movements,  are  the  delight  of  the  intelligent 
seashore  visitor,  and  especially  of  the  naturalist. 

How  do  they  take  their  food  ?  The  movement  of 
their  tentacles  is  far  too  slow  for  this  purpose.     They 


NUTRITION    PROl'KR. 


343 


Fig.  22g. — Diagram  of  a  medusa:  nn, 
nen.-e;  w,  mouth;  si,  stomach;  rt, 
radiating  tubes. 


take  it  by  means  of  what  have  been  called  nettle  cells, 
or  stinging  cells,  or  thread  cells  (nematocysts),  or,  by 
Agassiz,  most  appropri- 
ately, /asso  cells.  They 
occur  in  clusters,  espe- 
cially on  the  tentacles. 
Their  sha[)e,  like  an  elec- 
tric lamp,  is  seen  in  Fig. 
230.  A.  Examined  with 
the  microscope,  they  are 
seen  to  contain — like  an 
electric  lamp  —  a  fine 
thread  coiled  within.  If 
the  animal  be  irritated 
the  long  thread  flashes  out  like  lightning  (Fig.  230,  B) 
and  its  extremity  pierces,  discharges  a  poison,  and  par- 
alyzes its  prey,  which  is 
then  slowly  brought  to 
the  mouth  and  swallow'ed. 
The  effect  of  these  sting- 
ing cells,  with  their  in- 
visible threads  charged 
with  poison,  is  so  power- 
ful that  handling  these 
animals  will  produce  pain- 
ful inflammation  of  the 
hands. 

The  mouth  is  the  open- 
ing at  the  end  of  the  pro- 
boscis (Fig.  229),  and  the 
stomach — often  called  the 
oesophagus  —  the  hollow 
proboscis  itself.  The  food 
is  taken  by  the  mouth,  and  retained  in  the  stomach  until 
digested.     Whatever  is  indigestible  is  rejected  through 


F'iG.  230. — Lasso  cell  :  A,  in  passive 
state  ;  B,  with  the  thread  discharg- 
ing. 


344 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


the  mouth,  and  the  digested  food  is  then  circulated  by 
ciliary  currents  through  all  the  canals  of  the  body  (chy- 
miferous  canals),  and  thence  directly  absorbed  into  the 
tissues. 

Polyp. — Another  excellent  example  is  the  polyp,  or 
actinia,  or  sea  anemone.     The  well-known  appearance  of 


Fig.  231. — Simplified  figure  of  an  actinia. 

this  animal  is  shown  in  Fig.  231,  and  the  interior  struc- 
ture in  Fig.  232.  Imagine  a  short,  hollow,  fleshy  cyl- 
inder (body  cavity),  with  a  disk  below  (the  foot  disk) 
and  a  disk  above  (the  mouth  disk).     The  upper  disk  is 


Fig.  232. — Ideal  section,  vertical  and  horizontal,  showing  structure  :  /,  ten- 
tacles;  J,  stomach  ;  //,  partitions. 


NUTRITION    PROPER. 


345 


surrounded  with  hollow  tentacles  opening  into  the  body 
cavity,  and  in  its  center  is  the  mouth,  opening  into  the 
stomach  or  gullet,  the  lower  end  of  which  opens  into  the 
general  cavity.  The  stomach  does  not  hang  free,  but  is 
held  firmly  in  its  place  by  partitions  running  from  the 
outer  wall  to  the  stomach,  and  dividing  the  general 
cavity  into  many  triangular  apartments.  Many  parti- 
tions do  not  reach  the  stomach.  Below  the  stomach  the 
partitions  end  in  free  scythelike  margins,  so  that  all  the 
triangular  apartments  are  in  communication  with  one 
another. 

Process  of  Digestion. — The  food  is  taken,  as  in 
medusae,  by  thread  cells,  is  put  into  the  mouth  and 
swallowed,  and,  partly  at  least,  digested  in  the  stomach. 
Whatever  is  indigestible  is  thrown  back  to  the  sea 
through  the  mouth.  The  partly  digested  food  is  then 
dropped  into  the  general  cavity,  its  digestion  completed, 
and  then  circulated  by  ciliary  currents  throughout  the 
whole  interior  cavity  even  to  the  extremity  of  the  hol- 
low tentacles,  and  directly  absorbed  and  appropriated  by 
the  tissues. 

PROTOZO.\. 

Here,  again,  we  find  a  great  step  downward.  Thus  far 
we  have  found  a  circulating  fluid,  although  in  the  coelen- 
terates  it  is  not  blood  but  digested  food,  and  circulated 
not  by  the  mechanical  action  of  a  heart,  but  by  ciliary 
currents.  But  now  we  find  no  circulating  fluid  of  any 
kind.  The  food  is  digested  and  at  once  appropriated 
by  the  living  protoplasm.  But  even  in  protozoa  we  find 
two  grades.  In  the  Infusoria  (Fig.  233)  the  mouth  is 
surrounded  by  cilia,  and  particles  of  all  kinds  are 
brought  there  by  ciliary  currents.  If  any  are  suitable 
for  food  they  are  carried  down  into  the  stomach ;  if 
not,  they  are  rejected  and  whirled  away  by  the  same 
current.     In  RJiizopods  there  is  neither  mouth  nor  stom- 


346 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


ach  as  a  permanent  organ.     Food  touching  the  body  any- 
where is  ingulfed.    The  semifluid  protoplasm  flows  around 

It,  takes  it  in,  and  digests  it. 
Whatever  is  indigestible  is 
thrown  out  again,  and  the  di- 
gested part  at  once  appropri- 


M 


,-s? 


") 


Chr 


Fig.  233. — Paramcecum.  The  ar- 
rows show  the  course  of  the  food 
until  discharged. 


Fig.  234. — Araoebaproteus:y^,  food 
bodies  ;  cv,  contractile  vesicle ; 
K,  nucleus.     (After  Leidy.; 


ated  and  assimilated  (Fig.  234);  see  also  Fig.  156,  page 
240).  These  lowest  animals  have  neither  tissues  nor 
organs  of  any  kind,  but  extemporize  these  as  wanted — 
whether  locomotive  organs,  prehensile  organs,  mouth, 
or  stomach. 

It  is  from  such  low  beginnings  that,  it  is  believed,  all 
the  types  of  animal  structure  have  been  gradually  differ- 
entiated by  a  process  of  evolution  through  all  geological 
times. 


CHAPTER    III. 

BLOOD    SYSTEM. 

Returning  again  to  man,  we  have  now  carried  the 
food  to  the  blood.  We  therefore  take  next  the  blood 
system.  But  first  of  all  we  must  say  something  about 
the  blood  itself. 

SECTION    I. 
The  Blood. 

Blood  is  an  intensely  red  fluid,  of  a  slightly  viscid 
feel  and  a  faintly  nauseous  smell  and  taste.  To  the 
naked  eye  it  is  quite  homogeneous,  but  under  the  micro- 
scope it  is  easily  seen  to  consist  of  a  multitude  of  solid 
red  particles  or  globules  floating  in  an  almost  colorless 
liquid.  The  liquid  part  is  called  plasma.  The  color  is 
wholly  due  to  the  globules.  The  quantity  of  this  fluid 
in  a  healthy  man  is  about  one  and  a  half  gallon. 

r.  The  Globules. — These  are  of  two,  possibly  three 
kinds,  viz.  :  (a)  the  red  globules  (Fig.  235) ;  {b)  the  white 
or  colorless  globules ;  and,  doubtfully,  (r)  the  blood  plates. 
Of  these  the  red  are  far  the  most  numerous  and  conspic- 
uous, and  are  therefore  taken  first. 

{a)  Red  Globules. — These  in  size  are  about  ■g-s'oTr  '"*^h 
(tIt  ni'llin^etre)  in  diameter.  In  shape  they  are  flattened 
circular  disks,  a  little  depressed  in  the  middle,  their 
thickness  being  about  one  quarter  the  diameter  of  the 
disk.  Their  immense  number  may  be  shown  thus  : 
From   the  size  given   above  it  is  evident  that  it  would 

347 


348    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


take  ten  million  to  cover  a  square  inch  ;  but  on  account 
of  their  flatness  it  would  take  more  than  twelve  thou- 
sand to  make  a  pile  an  inch  high.  Thus  a  cubic  inch  of 
blood  globules  would  contain  one  hundred  and  twenty 
thousand  million  globules.  Now  it  has  been  estimated 
that  the  actual  number  in  a  cubic  inch  of  blood  is  about 
seventy  thousand  millions.  Therefore  considerably  more 
than  half  of  the  blood  consists  of  these  globules. 

Behavior  of  Blood. — If  we  watch  fresh-drawn  blood 
with  a  microscope  under  a  glass  cover  we  observe  that 
the  disks  run  together  and  pile  on  one  another  like  a 

pile  of  coins  thrown  down 
(Fig.  235).  This  is  the  re- 
sult of  coagulation.  If  a 
little  water  on  the  finger  is 
touched  to  the  side  of  the 
cover,  so  as  to  be  drawn 
in  and  mingled  with  the 
blood,  then  the  globules 
become  round  like  mar- 
bles, and  roll  around  freely 
in  the  current.  They  have 
imbibed  the  water  and  be- 
come swollen.  If  the  fluid 
in  which  they  float  should 
dry  away  ever  so  little, 
they  shrivel  and  become 
irregular  in  shape  (Fig.  235  ;').  The  same  change  takes 
place  if  a  strong  solution  of  salt  be  added.  The  salt 
draws  water  out  of  the  globules  and  shrivels  them.  But 
if  a  weak  solution  of  salt  be  added  it  prevents  coagu- 
lation, and  at  the  same  time  does  not  alter  the  shape. 
The  disklike  shape  may  be  thus  examined  at  leisure. 

Structure. — The  red  globules  are  not  cells,  but  solid 
masses,  a  little  softer  in  the  center. 


Fig.  235. — Blood  globules  :  r  rr,  red 
globules  ;  /  /,  white  globules  or 
leucocytes. 


BLOOD    SYSTEM. 


349 


(i>)  White  Globules  {^Leiicocytes). — These  are  much  fewer 
in  number,  being  on  an  average  only  about  one  in  eight 
hundred.  They  differ  from  the  red  in  color,  being  color- 
less ;  in  size,  being  a  little  greater  in  diameter  ;  in  shape, 


7         ^/      v::^^/ 

Fig.  236. — Different  forms  of  leucocytes. 

being  normally  spherical  instead  of  disk-shaped  ;  in 
structure,  being  distinctly  nucleated j  and,  last  and  most 
important  of  all,  in  being  living  cells,  endowed  with 
amoiboid  movement — in  fact,  in  having  all  the  properties 
and  capacities  of  living  protozoa,  crawling  about  like 
living  things.  Some  of  the  shapes  which  they  take  on 
are  shown  in  Fig.  236. 

(c)  Blood  Plates. — These  are  very  much  smaller  than 
either  of  the  others.  They  are  difficult  of  demonstra- 
tion, and  their  function  is  doubtful.  In  fact,  they  are 
believed  by  some  investigators  to  be  not  true  blood 
elements  at  all,  but  only  the  disintegrated  remains 
of  wornout  white  corpuscles.  This  is  still  under  dis- 
cussion. 

Chemical  Composition. — The  globules  of  blood 
seem  to  be  composed  of  an  albuminoid  stroma,  colored 
in  the  case  of  the  red,  with  another  peculiar  albuminoid 
substance — hczmoglobin.  This  substance  contains  a  nota- 
ble quantity  of  iron,  and  has  a  remarkable  relation  to 
oxygen.  It  readily  takes  up  o.xygen,  and  as  readily 
gives  it  up  again,  and  in  doing  so  changes  color.  In 
an  oxidized  condition  (oxyhaemoglobin)  it  is  intensely 
scarlet  red.  In  the  deoxidized  condition  it  is  darkpur- 
plish  red. 


350 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


2.  Plasma. — The  liquid  in  which  the  globules  swim 
(plasma)  is  a  solution  of  fibrin^  albumen,  and  salts,  with 
often  a  trace  of  sugar  and  fats. 

Coagulation  of  Blood, — If  blood  be  drawn  and 
allowed  to  stand  it  coagulates.  This  is  produced  by  the 
solidification  of  the  fibrin.  All  albuminoids  pass  more 
or  less  easily  from  the  liquid  to  the  solid  state.  Albu- 
men, or  white  of  tgg,  is  solidified  by  heat  at  i6o°,  or  by 
alcohol ;  casein  of  milk  is  solidified  by  acid  and  heat  at 
70°  to  80°  or  less.  Fibrin  solidifies  much  more  easily; 
exposed  to  air  or  touching  any  foreign  body,  it  solidifies 
at  once.  The  causes  and  conditions  of  the  solidification 
of  fibrin  are  still  under  discussion. 

If  blood  be  drawn  and  allowed  to  stand,  as  already 
said,  it  coagulates  first  into  a  tremulous  jelly  by  the 
solidification  of  the  fibrin.  If  it  continues  to  stand, 
gradually  the  fibrin  contracts  more  and  more  firmly,  and 
a  liquid  is  squeezed  out.  The  whole  mass  thus  separates 
into  two  parts,  clot  and  serum.  The  clot  consists  of 
the  solidified  fibrin  and  the  entangled  globules.  The 
serum  consists  of  a  solution  of  albumen  and  salts,  with 
perhaps  a  trace  of  sugar.  This  is  shown  in  the  follow- 
ing schedule  : 

Blood 
Fresh. 

Globules 


^  Red. 

Coagulated. 

'/  \Vhite. 

\  Clot. 

r  Fibrin. 

) 

Albumen. 

1 

1 

'  Salts. 
Sugar,  a  trace. 
Fat,  a  trace. 

1 

I'  Serum. 

1 

J 

Plasma,  solution  of.  . 


In  fever  blood  coagulates  more  slowly.  In  coagu- 
lation the  globules  have  time  to  settle  a  little  before 
the  solidification  of  the  fibrin,  leaving  thus  the  char- 
acteristic buff  coat  of  fevers  on  the  top.     If  salt  be  added 


BLOOD    SYSTEM. 


351 


to  fresh  blood  it  prevents  coagulation,  and  the  globules 
will  settle  to  the  bottom.  If  freshly  drawn  blood  be 
stirred  or  whipped  with  a  bundle  of  twigs  or  wires,  the 
whole  of  the  fibrin  coagulates  on  the  wires  or  twigs  as 
fleshy  strings,  and  may  be  withdrawn.  The  defibrinated 
blood  will  no  longer  coagulate.  This  is  important,  be- 
cause it  is  necessary  sometimes  to  transfuse  the  blood 
of  one  person  into  the  veins  of  another,  or  even  the 
blood  of  an  animal  into  the  veins  of  a  man.  If  so, 
then  the  blood  so  transfused  must  not  be  liable  to 
coagulation. 

Functions  of  the  Blood. — i.  Plasma. — The  plasma 
may  be  regarded  as  essentially  the  finished  result  of 
albuminoid  food,  although  it  contains  in  small  quantities 
many  other  substances  for  use  or  for  elimination  by  ex- 
cretory organs.  It  is  essentially  the  peptones,  sangui- 
fied,  vitalized,  and  ready  for  use.  Its  fundamental 
function,  then,  is  tissue  building — i.  e.,  repair  and  growth 
of  tissue.  Its  peculiar  property  of  easy  change  from 
liquid  to  solid  form  is  eminently  adapted  for  this  pur- 
pose. Its  liquid  condition  is  necessary  for  circulation ; 
its  solid  condition  is  necessary  for  making  tissue.  Fur- 
ther than  this,  however,  the  whole  process  of  assimila- 
tion, or  change  from  blood  to  tissue,  is  still  enveloped  in 
mystery. 

2.  Red  Globules. — The  function  of  these  is  better  un- 
derstood than  any  other  part  of  the  blood.  They  are 
undoubtedly  carriers  of  oxygen  from  the  air  to  the  tis- 
sues for  combustion  of  their  waste.  This  it  does  by  vir- 
tue of  the  property  of  haemoglobin,  already  mentioned. 
This  substance  takes  oxygen  from  the  air  in  respiration, 
becomes  oxidized  as  oxyhaemoglobin,  is  carried  to  the 
tissues,  and  there  gives  up  its  oxygen  for  the  combus- 
tion of  waste.  It  is  easily  seen,  therefore,  why  an  abun- 
dance of  red  globules  is  necessary  for  health  and  vigor. 
24 


352 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


3.  White  Globules. — The  function  of  the  white  globules 
is  much  more  obscure.  In  some  way  little  understood 
they  seem  to  vitalize  the  blood,  for,  as  already  seen, 
they  are  really  living  protozoa.  It  will  be  remembered 
(page  329)  that  as  soon  as  these  enter  the  chyle  in  the 
mesenteric  glands  this  fluid  becomes  endowed  with  the 
property  of  coagulation.  Again,  it  is  believed  that  these 
globules  act  as  scavengers  of  the  blood,  removing  mi- 
crobes and  other  offending  substances  by  inclosing  and 
devouring  them  after  the  manner  of  rhizopods  (page 
346).  Again,  as  already  seen,  these  corpuscles  crawl 
about  and  change  their  shape  like  amoeba.  When  the 
blood  stagnates,  as  in  inflammation,  they  actually  squirm 
themselves  through  the  thin  walls  of  the  capillaries,  and 
wander  about  in  the  tissues;  and,  in  case  of  breaking 
down  of  the  tissues  and  the  formation  of  pus,  they  prob- 
ably become  the  characteristic  pus  corpuscles. 

Origin  and  Life  History  of  Blood. — i.  Plasma,  as 
already  explained,  is  essentially  the  sanguified  and  vital- 
ized peptones.  Sugar  is  also  taken  up  into  the  blood, 
and  in  so  far  as  it  remains  there  is  a  constituent  of  the 
plasma  ;  but  normally  there  is  only  a  trace  of  sugar  in 
the  blond  of  \.\\&  general  circulation,  because  it  is  quickly 
burned  up.  We  shall  speak  of  this  more  fully  in  another 
place. 

2.  White  Globules,  or  Leucocytes. — We  have  already 
seen  that  these  are  formed  in  the  mesenteric  glands; 
but  these  are  only  lymphatic  glands  situated  in  the  mes- 
enterv.  Leucocytes  are  formed  in  all  lymphatic  glands 
wherever  situated  and  at  all  times,  whether  chyle  be 
passing  through  them  or  not.  They  are  also  formed  in 
the  spleen,  for  the  blood  of  the  splenic  vein  before  it 
enters  the  vena  porta  is  ten  to  twenty  times  richer  in 
leucocytes  than  that  of  the  general  circulation.  It 
seems  certain,  then,  that  the  leucocytes  are  formed  in 


BLOOD    SYSTEM.  353 

the  lymphatic  glands  and  in  the  spleen,  and  that  the 
blood  receives  them  while  passing  through  these  organs. 
How  they  disappear  is  not  certainly  known.  Possibly 
they  break  up  into  so-called  blood  plates. 

3.  Red  Globules. — These  are  constantly  changing  ; 
they  have,  like  all  living  things,  a  delinite  life  history; 
they  are  born,  mature,  decay,  and  die.  If  the  body  is 
depleted  of  blood  it  quickly  recovers  its  supply.  If  the 
blood  of  another  animal  be  transfused  into  the  veins  of 
a  man,  at  first  two  kinds  of  globules  may  be  seen  in  the 
blood,  but  as  time  goes  on  the  animal  globules  decrease 
and  the  human  increase  until  only  the  human  remain. 
The  red  globules  are  therefore  being  renewed  all  the 
time.  Where  do  they  come  from  ?  At  one  time  they 
were  thought  to  be  transformed  leucocytes;  but  this  is 
probably  not  true.  It  is  now  believed  that  they  are 
formed  in  the  red  marrow  of  the  bones,  and  perhaps  also 
in  the  spleen,  by  the  division  of  certain  large  cells  ob- 
served there.  This  is  their  birthplace.  Their  active  life 
is  in  the  circulation,  doing  their  work  of  o.xygen  carrv- 
ing.  Their  death  place  is  probably  the  liver  and  the 
spleen. 

Briefly,  then  : 

Leucocytes  are  born  in  the  lymphatics  and  the  spleen. 

Red  globules  are  born  in  the  bones  and  the  spleen. 

Red  globules  die  m  the  liver  and  the  spleen. 

It  would  seem,  then,  that  the  spleen  has  many  and 
important  functions,  but  that  it  shares  all  these  with 
other  organs.  May  not  this  explain  the  singular  fact 
that,  although  so  important,  yet  it  has  been  extirpated 
in  the  dog  without  immediate  serious  injury  to  health. 

Now  the  blood  goes  everywhere,  touches  every  tissue 
and  every  cell,  (i)  It  carries  food,  and  thus  becomes 
the  all-nourisher.  (2)  It  takes  up  all  waste  and  carries 
it  to  the  appropriate  organ  of  elimination,  and  thus  be- 


354    PHYSIOLOGY  AND    MORPHOLOGY   OP'   ANIMALS. 

comes  the  all-purifier.  (3)  It  takes  oxygen  to  every 
tissue  to  consume  waste  and  food  by  combustion,  and 
thus  generates  heat  and  force,  and  thus  becomes  the 
all-warmer  and  energizer.  (4)  It  acts  also  as  a  reservoir, 
especially  for  food.  The  food  taken  to-day  is  not  used 
to-day,  but  the  blood  is  drawn  upon  for  heat  and  force 
and  tissue  repair  and  again  resupplied.  The.  blood  is 
like  a  lake  in  irrigation  or  a  bank  in  currency.  (5)  To 
a  much  less  extent  it  is  also  a  reservoir  for  waste;  to  a 
less  extent  because  the  urgency  of  waste  removal  is 
much  greater. 

Comparative  Morphology  of  Blood. — In  all  ver- 
tebrates we  have  two  kinds  of  globules,  the  red  and  the 
white.  The  white,  or  leucocytes,  are  substantially  simi- 
lar in  all  ;  but  the  red  differ  in  size,  shape,  and  structure 
in  the  different  classes. 

1.  Mammalian  Blood. — The  blood  of  all  mammals 
(Fig.  237)  is  substantially  similar  to  that  of  man,  already 
described.  The  red  corpuscles  (globules)  are  all  small 
circular .,no7i- nucleated  (X\sks.  The  only  exception  in  shape 
is  in  the  camel,  in  which  they  are  elliptic  instead  of  cir- 
cular, but  otherwise  they  are  as  stated  above.  In  size 
they  bear  no  apparent  relation  to  the  size  of  the  animal ; 
those  of  a  mouse,  on  the  one  hand,  and  of  an  elephant, 
on  the  other,  not  differing  in  any  marked  degree  from 
that  of  man.  Even  expert  microscopists  are  in  doubt 
whether  the  blood  of  a  dog  can  be  certainly  distin- 
guished from  that  of  a  man. 

All  mammals  (except  monotremes)  are  viviparous  or 
young-bearing.  Therefore  this  style  of  blood  may  be 
called  viviparous  vertebrate  blood. 

2.  Birds,  Reptiles.,  Amphibians,  and  Fishes. — All  other 
classes  of  vertebrates — viz.,  birds,  reptiles,  amphibians, 
and  fishes — have  blood  globules  differing  in  size,  shape, 
and  structure  from  those  of  mammals,  but  similar  to  one 


BLOOD   SYSTEM. 


>s 


another  in  s/iajyi"  and  structure,  though  varying  in  size. 
In  shape  they  are  all  elliptic;  in  structure  they  are  nucle- 
ated j  in  size  they  vary  from  nearly  the  size  found  in 
mammals  in  certain  fishes  to  very  many  times  that  size 
in    some  amphibians.     The   largest   are   found  in   some 


Mai 


Bird 


tep 


Ol     O  6 


7ish 


Fig.  237. — Blood  of  different  classes  of  vertebrates,  showing;  comparative 
sizes.  All  are  equally  mag:nified  :  /«,  man  ;  el,  elephant  ;  ms,  mouse  ; 
W7,  musk  deer  ;  lib,  humming:-bird  ;  /,  pigeon  :  ost,  ostrich  ;  sn,  snake  ; 
/,  toad  ;  //',  proteus  ;  /,  perch  ;  pk,  pike  ;  sh^  shark. 


tailed  amphibians,  such  as  \.\\^ proteus,  in  which  they  are 
ten  times  the  diameter  or  one  hundred  times  the  surface 
area  of  those  of  man  (Fig.  237). 

All  these  classes  of  vertebrates  are  egg-laying. 
Therefore  blood  containing  this  style  of  red  corpuscles 
may  be  called  oinparous  vertebrate  blood. 

All  vertebrate  blood  (unless  we  except  that  of  the 


356   PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 

amphioxus,  which  is  very  doubtfully  a  vertebrate)  is  red. 
Below  this — i.  e.,  in  invertebrates — the  blood  is  not  red.* 

3.  Higher  Invertebrates. — Among  these  we  include 
arthropods,    worms,    mollusca,    and    echinoderms — i.  e., 

all  above  coelenterates ;  in  other 
words,  all  that  have  a  blood  sys- 
tem and  a  true  blood  at  all.  In 
these  the  red  corpuscles  are 
wanting;  only  white  corpuscles 
are  found  (Fig.  238).  In  these, 
Fig.  238.— Blood  corpuscles      therefore,  the  blood  is  colorless 

of  a  crustacean.     (After  „,.,.,  ,      i, 

Gegenbaur. )  or  nearly  so.     1  his  kind  we  shall 

call  mvertebrate  blood. 

4.  Civlenterates. — These  include  medusae  and  polyps. 
In  these,  as  already  said  (page  342),  we  have  no  true 
blood  differentiated  from  digested  food,  nor  blood  sys- 
tem differentiated  from  the  digestive  system.  The  di- 
gested food  carried  by  ciliary  circulation  is  the  nutrient 
fluid.  This,  therefore,  may  be  cMtd  food  blood,  or  coelen- 
terate  blood. 

5.  Protozoa. — In  these  there  is  no  circulating  fluid  of 
any  kind. 

The  following  schedule  briefly  expresses  these  facts  : 

„,      ]    r  I.   Mammalian         )  ^^      ,  ,  ,         , 

lilood    \  .   .  J- Blood  —  red,  non-nucleated, 

red.     ■  viviparous.        \ 

(2.  Oviparous  vertebrate  blood  —  red,  nucleated — birds,  rep- 
tiles, amphibians,  and  fishes. 
Blood    /  3.  Invertebrate      blood  —  white,      nucleated  —  arthropods, 
color-  \  worms,  mollusca,  echinoderms. 

less.     '  4.  Ccelenterate    blood — white,    no    globules,   food     blood — 
medusas,  and  polyps. 
5.  Protozoa  —  no  circulating  fluid. 

Embryonic  Development  of  Blood. — Some  of  the 
above  phases  are  found  in  the  embryonic  development 

*  Some  worms  have  a  kind  of  red  "blood. 


BLOOD   SYSTEM. 


JO/ 


of  mammals,  and  even  of  man.  (i)  In  the  earliest  stages 
of  egg  development — i.  e.,  before  the  organs  are  formed 
— there  is,  of  course,  no  circulating  fluid  of  any  kind, 
no  blood  or  blood  system.  This  corresponds  remotely 
to  the  lowest  cell-aggregate  animals  in  which  there  is 
yet  no  circulation.  (2)  The  next  thing  observed  is  the 
liberation  of  nucleated  tissue-cells  along  certain  lines  and 
the  oscillatory  movement  of  the  liberated  cells  along 
these  lines.  This  is  the  beginning  of  blood  vessels  and 
of  a  blood  which  has  colorless  nucleated  cells  like  the  blood 
of  invertebrates.  A  heart  is  added  later,  by  the  develop- 
ment of  a  part  of  the  vascular  system.  (3)  The  nucleated 
corpuscles  become  reddened,  and  we  have  now  nucleated 
red  corpuscles  like  the  blood  of  the  lower  vertebrates. 
(4)  The  small  non-nucleated  round  disks  begin  to  appear 
among  the  others,  which  gradually  disappear,  and  we 
have  true  mammalian  blood. 

It  would  seem  now  that  in  logical  order  we  ought  to 
take  up  the  circulatory  system.  But  the  course  of  the 
blood  is  so  wholly  determined  by  its  distribution  through 
the  respiratory  organs  that  it  would  be  impossible  to 
understand  that  course,  especially  its  comparative 
morphology,  without  a  previous  knowledge  of  the 
organs.  These,  therefore,  must  be  first  taken  up.  On 
the  other  hand,  the  function  of  respiration  is  wholly  a 
katabolic  process  and  the  most  important  of  all  these 
processes,  and  must  be  treated  along  with  these  in  the 
fourth  chapter.  Therefore,  our  plan  will  be  to  take  up, 
first,  the  morphology  of  the  respiratory  organs  in  vertebrates, 
then  the  morphology  of  the  circulatory  system,  also  in 
vertebrates ;  then  the  morphology  of  circulation  and  res- 
piration together  in  the  invertebrates;  and,  finally,  the 
physiology  or  function  of  respiration — which  is  the  same 
in  all  animals.     This,  of  course,  will  be  treated  with  the 


358    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

katabolic  processes,  as  the  most  important  of  all  of  them. 
All  that  is  necessary  now  in  regard  to  the  respiratory 
function  is  only  to  remember  that  the  general  object  of 
respiration  is  the  aeration  of  the  blood,  and  thereby  the 
exchange  of  its  COg  for  oxygen  of  air. 


SECTION    II. 

Respiratory  Organs  among  Vertebrates. 

Respiratory  organs  are  of  two  general  kinds,  viz., 
lungs  and  gills.  The  one  kind  is  adapted  for  air  breath- 
ing,  the  other  for  water  breath- 
ing. The  general  plan  of  the 
one  is  a  complexly  /;;-folded  epi- 
thelial surface  ;  of  the  other  of 
a  complexly  <?«/-folded  epithe- 
lial surface  (Fig.  239).  In  both 
cases  alike  the  purpose  is  to  ex- 
pose as  large  a  surface  as  pos- 
sible to  the  oxygen  of  the  air  or 
the  water.  The  two  kinds  are 
not  homologous  ;  the  one  is  not 
transformed  into,  but,  as  already 
shown  (page  242),  substituted  for 
the  other.  We  take  first  lungs,  and,  of  course,  first  of 
all  the  lung-s  of  man. 


Fig.  239. — General  plan  of 
structure,  A,  of  lungs ; 
B,  of  grills. 


I.     LUNG    RESPIRATION. 

The  Lungs  of  Man. — The   lungs   are  by  far  the 

largest  organ  in  the  body,  although  in  tveight  they  are 
greatly  exceeded  by  the  liver.  Their  great  volume  is, 
of  course,  due  to  the  contained  air.  Their  grayish-pur- 
ple color  and  soft,  elastic  feel  is  well  known.  They  are 
a  double  organ,  being  divided  into  a  right  and  left  lung 
by  the  mediastinum. 


BLOOD   SYSTEM. 


359 


Structure. — We  have  already  said  (page  204)  that 
two  tubes  go  down  from  the  throat  into  the  body  cavity 
— the  posterior  one,  the  gullet,  to  the  stomach,  and  the 
anterior  one,  the  windpipe,  or  trachea,  to  the  lungs. 
The  one  is  soft  and  flaccid,  but  muscular;  the  other  is 
a  firm,  open  tube,  being  kept  open  by  a  series  of  carti- 
laginous rings.  This 
is  necessary  in  order 
that  the  air  may  come 
and  go  with  the  least 
resistance  possible. 
The  trachea  is  capped 
above  by  the  larynx, 
as  already  explained 
(page  204).  After  a 
course  of  about  five 
inches  it  divides  into 
two  great  branches, 
one  to  each  lung, 
called  the  bronchi. 
These  are  also  ringed. 
The  primary  bronchi 
are    subdivided    into 

secondary        bronchi.     Fig.    240.  —  Diafjram   showing   the   general 
J      ,  .      .  structure  of  the  lungs  of  man  :  L,  larynx  ; 

and    these   again  into  7-^,  trachea;  br,  bronchi. 

tertiary,    and    so    on 

until  the  tubes  become  of  capillary  fineness  and  corre- 
spondingly numerous.  This  is  shown  diagrammatically 
in  Fig.  240.  They  finally  terminate  in  minute  cells  or 
cellulated  cells  (Fig.  241).  The  ringed  structure  con- 
tinues, except  in  the  smallest  subdivisions  and  the  ter- 
minal cells.  The  terminal  cells  are  y^  to  ^^^  of  an  inch 
{\  to  \  millimetre)  in  diameter,  and  their  number  has 
been  estimated  as  600,000,000.  Now  conceive  this  mass 
of  finely  divided  tubes  and  terminal  cells,  ///^\/ through- 


360  PHYSIOLOGY   AND    MORPHOLOGY   OF    ANLMALS. 


out  with  epithelial  membrane — an  extension  of  the  mu- 
cous membrane  of  the  mouth  and  throat — webbed  to- 
gether with  loose  connective  tissue  and  invested  with 
serous  membrane,  and  we  shall  have  a  sufficiently  clear 
idea  of  the  general  structure  of  the  lungs. 

The  obvious  purpose  of  all  this  is  to  expose  as  much 
surface  as  possible  to  the  oxygen  of  the  air.  The 
minuter  the  ramification,  the  smaller  the  terminal  cells; 
or  the  finer  the  sponge,  the 
larger  will  be  the  surface. 
The  area  of  the  epithelial 


Fig.  241. — Termination  of  a  capil- 
lary bronchus  :  a,  bronchus  ;  6, 
cell ;  c,  cellule. 


Fig.  242.  —  Showing;  the  capillary 
blood  vessels  (b)  ramifying  on 
the  surface  of  the  cells  (a). 


surface  exposed  in  the  lungs  has  been  variously  esti- 
mated from  four  hundred  to  fifteen  hundred  square  feet. 
Now  beneath  this  extensive  epithelial  surface  the  blood 
vessels  ramify  most  minutely  and  exchange  with  the  air 
COo  for  oxygen  (Fig.  242). 

But  to  make  this  exchange  effective  both  the  air  and 
the  blood  must  be  in  constant  circulation.  When  the 
blood  has  discharged  its  COg  and  taken  in  its  supply  of 
O  it  must  get  out  of  the  way  for  other  blood  to  do  the 
same.  Similarly,  when  the  air  has  given  up  its  O  and 
taken   in   its   supply  of  COo,  it   must  move  on  and  give 


BLOOD   SYSTEM. 


361 


place  for  other  air.  In  the  case  of  the  blood  the  circu- 
lation is  kept  up  by  the  mechanical  action  of  the  licart ; 
in  the  case  of  the  air,  by  the  mechanical  action  of 
breathing. 

Mechanics  of  Breathing. — The  body  cavity  is 
divided  by  a  thin  transverse  partition  into  an  upper 
chamber,  the  thorax,  and  a  lower  chamber,  the  abdomen. 
In  the  one  is  found  the  lungs  and  heart,  in  the  other  the 
stomach,  spleen,  liver,  intestines,  kidneys,  etc.  The  dia- 
phragm is  not  3l  plane,  or  it  could  not  be  used  for  respira- 
tion. It  is  deeply  concave  below,  or  domelike.  It  is  also 
a  muscular  partition,  the  fibers  radiating  from  a  clear 
membranous  space  in  the  middle — the  skylight  of  this 
dome — in  all  directions  and  taking  hold  of  the  walls 
of  the  body  cavity  all  around.  Contraction  of  these 
fibers  brings  down  the  dome  and  flattens  it.  The  arch 
of  the  dome  is  filled  below  by  the  stomach  and  spleen 
on  the  left  and  the  liver  on  the  right.  These  in  their 
turn  rest  on  the  mass  of  convoluted  intestines,  and  the 
whole  is  supported  by  the  abdominal  walls.  Above  the 
diaphragm  the  concave  lower  surface  of  the  lungs  rests 
directly  on  the  upper  convex  surface  of  the  dome. 
Although  in  contact  with  the  viscera  above  and  below, 
the  diaphragm  is  not  united  with  them. 

Now,  the  thoracic,  like  the  abdominal,  cavity  is  lined 
with  a  smooth  serous  membrane.  In  the  abdomen  it  is 
called  the  peritonceum,  in  the  thorax  the  pleura.  Like 
the  abdominal,  so  the  thoracic  cavity  is  a  closed  cavity — 
i.  e.,  the  pleura  lines  the  ribs,  etc.,  and  is  then  reflected 
to  form  the  investing  membrane  of  the  lungs  and  heart. 
If  it  could  be  successfully  dissected  off  it  would  form  a 
continuous  bag  without  a  hole  in  it.  The  manner  in 
which  the  pleura  lines  the  thorax,  is  then  reflected  over 
the  lungs  as  its  investing  membrane,  and  then  between 
the  two  lungs  as  the  mediastinum,  is  shown  in  Figs.  243 


362    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


and  244.  For  greater  clearness  there  is  a  space  between 
the  lungs  and  the  walls.     In  reality  they  are  in  contact. 

This  description  is  necessary  to  make  clear  the  me- 
chanics of  breathing,  which  we  now  proceed  to  explain. 

Breathing  consists  in  alternate  expansion  and  con- 
traction of  the  thoracic  cavity  by  means  of  the  respira- 


FiG.  243. — Vertical  section  through 
the  lungs,  showing  the  relation 
to  the  pleura  :  /r,  trachea  ;  b7\ 
bronchi ;  cpl,  costal  pleura  ;  ///, 
pulmonic  pleura. 


Fig.  244. — Horizontal  section  through 
the  thorax  :  /r,  trachea  ;  H,  heart ; 
pc,  pericardium ;  ///,  pulmonic 
pleura  ;  c//,  costal  pleura. 


tory  muscles.  The  lungs  are  perfectly  passive  in  the 
operation.  To  illustrate :  Suppose  we  take  a  hand 
bellows  and  arrange  a  bladder  within  so  as  to  connect 
with  the  nozzle,  with  no  opening  into  the  bellows,  but 
only  into  the  bladder,  through  the  nozzle.  Now,  on 
expanding  the  bellows  the  air  rushes  in  through  the 
nozzle  into  the  bladder  and  expands  it,  but  no  air  enters 
the  cavity  of  the  bellows.  On  shutting  up  the  bellows 
the  air  is  again  squeezed  out  from  the  bladder  through 
the  nozzle.  In  all  these  movements  the  exterior  sur- 
face of  the  bladder  and  the  interior  surface  of  the  bel- 
lows never  break  contact.  But  if  there  be  an  opening  on 
the  side  of  the  bellows  the  air  will  rush  in  there  and  not 
fill  the  bladder. 

Application    to    Breathing. — So    the    lungs    are 
placed  in   the  thorax  as  the  bladder  in   the  bellows.     If 


BLOOD   SYSTEM. 


363 


the  thorax  expands,  air  rushes  in  through  the  nostrils 
into  the  lungs,  expanding  them  and  keeping  them  in  con- 
tact with  the  ribs.  If  the  thorax  contracts,  it  squeezes 
out  the  air  through  the  nostrils.  But  if  there  be  an 
opening  into  the  thorax  (by  wound),  then,  on  expand- 
ing the  thorax,  the  air  rushes  in  there,  gets  into  the  cav- 
ity instead  of  into  the  lungs,  and  the  lungs  do  not  fill. 

Mode  of  expanding  and  contracting  the 
Thorax. —  rhis  is  done  in  two  ways — viz.,  by  the  ribs 
and  by  the  diaphragm.  There  are  therefore  two  kinds 
of  respiration — viz.,  costal  or  thoracic,  and  diaphrag- 
matic or  abdominal. 

Costal  Respiration. — The  ribs  do  not  run  straight 
around  the  chest  horizontally,  but  slope  downward  all 
the  way.  The  simple  lifting  of  the  ribs,  therefore, 
increases  the  whole  transverse  section  of  the  chest. 
Now,  between  the  ribs 
and  connecting  them  there 
are  two  sheets  of  muscu- 
lar fibers,  external  and  in- 
ternal. The  fibers  of  the 
external  sheet,  ex,  run 
obliquely  downward  and 
forward,  those  of  the  in- 

■  iTlt 


,i,X. 


Fig.  245. — Showing  two  ribs 
and  the  intercostals  be- 
tween :  ex,  external,  and 
/'«/,  internal  sheet. 


Fig.  246. — Diagram  illustrating  action  of 
the  intercostal  muscles  :  a  a,  fiber  of 
exterior  sheet  when  passive  ;  a  a\ 
when  contracted,  b'  b'  a  fiber  of  inte- 
rior sheet  when  stretched,  and  b  b 
when  contracted. 


terior  sheet,  int,  downward  and  backward  (Fig.  245). 
The  external  sheet  raises  the  ribs  and  expands  the  chest, 
the  internal  sheet  pulls  down  the  ribs  and  contracts  the 


364    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

chest.  Why  this  must  be  so  is  shown  in  diagram,  Fig. 
246.  If  I,  2  and  3,  4  be  two  ribs  connected  with  the 
backbone  behind  and  the  breastbone  in  front,  and  there- 
fore compelled  to  nwi'e  together  and  in  the  natural  slop- 
ing position,  and  if  a  a  represent  one  fiber  of  the  exter- 
nal sheet,  it  is  evident  that  if  this  fiber  contracts  it  will 
elevate  both  ribs,  because,  although  it  pulls  one  up  and 
the  other  down,  yet  the  leverage  on  3,  4,  by  which  it 
pulls  up,  is  greater  than  that  on  i,  2,  by  which  it  pulls 
down  ;  and  since  they  must  move  together,  the  pair  will 
move  up.  But  upward  movement  expands  the  chest.  Or, 
put  it  another  way  :  Any  position  of  the  ribs  will  be 
assumed  which  shortens  the  fiber  a  a  to  a'  a' .  For  the 
same  reason  contraction  of  the  fibers  of  the  interior 
sheet  {b'  b')  will  depress  the  ribs  and  contract  the  chest, 
because  here  the  downward  pulling  acts  with  the  greater 
leverage  on  i,  2'.  Therefore  thoracic  or  costal  respira- 
tion is  accomplished  by  alternate  contraction  of  the  ex- 
terior and  interior  intercostal  muscles.  The  one  pro- 
duces /aspiration,  the  other  (?jcpiration. 

Diaphragmatic  or  Abdominal  Respiration.— This 
is  accomplished  by  the  alternate  contraction  of  the  dia- 
phragm and  the  abdominal  muscles.  When  the  muscular 
fibers  of  the  diaphragm  contract,  the  high  arch  is  brought 
down  and  the  dome  flattens.  This  increases  the  vertical 
diameter  of  the  thorax  as  the  intercostal  contraction 
does  the  transverse  diameter.  But  the  descent  of  the  dia- 
phragm presses  on  the  stomach  and  liver,  and  these  again 
on  the  intestines  on  which  they  rest;  and  these  in  turn 
press  outward  on  the  abdominal  wall,  which  therefore  pro- 
trudes. This  is  /;;spiration.  The  stretched  abdominal 
muscles  reacting,  by  contraction,  press  on  the  intestines, 
and  these  upward  on  the  stomach  and  liver,  and  these 
in  turn  lift  the  dome  of  the  diaphragm  and  press  on  the 
lungs,  squeezing  out  the  air.     This  is  i?Apiration. 


BLOOD   SYSTEM. 


365 


Fig.  247. — Diagrams  showing;  the  different  kinds 
of  respiration  :  A,  pure  thoracic  ;  H,  pure  ab- 
dominal;  C,  mixed.  1  he  dotted  lines  repre- 
sent the  expanded  condition. 


These  two  kinds  of  respiration  may  be  considered, 
and  indeed  may  be  used  separately.  Fig.  247,  A,  is 
a  diagram  repre- 
senting pure  tho- 
racic, and  Fig. 
247,  B,  pure  ab- 
dominal respira- 
tion. But  the  two 
are  usually  com- 
bined, and  the 
whole  body  cavity 
enlarges  and  con- 
tracts as  in  Fig. 
247,  C.  In  labored 
respiration       the 

thoracic  predominates;  in  quiet  respiration,  and  espe- 
cially in  sleep,  the  abdominal  predominates.  In  men 
there  is  a  slight  predominance  of  the  abdominal,  in 
women  of  the  thoracic. 

Thus,  then,  the  /V/spiratory  muscles  are  the  ^.rternal 
intercostals  and  the  diaphragm;  the  expiratory  mus- 
cles are  the  internal  intercostals  and  the  abdominal 
muscles. 

Coughirjg  and  sneezing  are  violent  convulsive  actions 
of  the  ^.vpiratorv  muscles.  It  is  a  little  singular  that  in 
popular  literature,  and  even  in  many  school  te.\t-books 
of  physiology,  these  should  be  so  often  referred  to  con- 
vulsive action  of  the  diaphragm.  Of  course,  this  is  im- 
possible since  the  diaphragm  is  an  /V/spiratory  muscle, 
and  coughing  and  sneezing  are  expulsive  efforts.  Every- 
body knows  that  constant  e.xcessive  coughing  will  pro- 
duce soreness  of  the  abdominal  muscles.  Hiccough  is  a 
spasmodic  action  of  the  diaphragm,  and  laughter  a  spas- 
modic alternate  action  of  the  inspiratory  and  e.xpiratory 
muscles,  with  the  latter  predominating. 


366 


PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 


COMPARATIVE     MORPHOLOGY     OF     VERTEBRATE     RESPIRA- 
TION. 

Mammals. — The  respiratory  organs  and  the  mode 
of  breathing  among  mammals  do  not  differ  from  those 
of  man.  All  mammals  have  a  diaphragm,  and  therefore 
both  kinds  of  respiration.  On  account  of  the  horizontal 
position  of  the  body,  of  course  the  body  cavity  is  divided 
into  an  anterior  and  posterior  instead  of  an  upper  and 
lower  chamber. 

Birds. — In  birds  we  find  large  lungs  of  minutely 
spongy  structure,  and  therefore  a  perfect  aeration  of  the 
blood.  They  are,  therefore,  hot-blooded  animals.  Yet 
birds  have  no  diaphragm.  This,  however,  is  of  little  dis- 
advantage, as  their  thorax  is  large  and  abdomen  small, 
and  their  thoracic  respiration  very  perfect.  It  is  interest- 
ing to  note,  however,  that  some  birds,  sijch  as  the  os- 
trich, seem  to  have  the  beginnings  of  a  diaphragm.  In 
these,  certain  muscular  fibers  arise  from  the  backbone 
and  lowest  ribs  behind  and  take  hold  on  the  lung  and 
pull  it  downward,  and  undoubtedly  assist  in  breathing. 

Reptiles. — Reptiles  have  no  diaphragm  ;  but  since 
their  ribs  surround  the  whole  body  cavity  to  the  hips. 


Fig.  248.— Skeleton  of  a  lizard  (Ne/oderma),  showing  how  the  ribs  surround 
the  whole  body  cavity. 


there  seems  to  be  no  use  for  a  diaphragm  in  them  (Fig. 
248).  Probably  the  shortening  of  the  rib  series,  in  order 
to  give  greater  freedom  to  the  loin  created  the  necessity 


BLOOD    SYSTEM. 


367 


of  a  diaphragm  in  mammals.  The  aeration  of  the  blood 
of  reptiles,  however,  is  very  imperfect,  for  reptiles  are 
cold-blooded.  This,  however,  is  due  not  so  much  to 
imperfect  respiration  as  to  the  coarseness  of  the  spongy 
structure  of  the  lungs,  and  therefore  the  smaller  surface 
of  contact  of  blood  with  air.  It  is  due,  also — as  we  shall 
see  hereafter — to  the  fact  that  only  a  part  of  the  blood 
is  exposed  to  the  air. 

Tortoise. — There  is  one  order  of  reptile  the  me- 
chanics of  whose  respiration  is  entirely  peculiar — viz.,  the 
tortoise.  Like  other  reptiles,  they  have  no  diaphragm, 
and  therefore  can  not  have  this  kind  of  breathing,  but 
neither  can  they  have  costal  breathing,  for  their  ribs  are 
immovably  consolidated  with  the  shell.  The  problem, 
then,  is  how  to  expand  the  body  cavity.  As  might  be 
expected,  therefore,  the  breathing  of  tortoises  and  turtles 
is  exceptional.  It  is  effected  partly  by  muscular  sheets 
which  arise  from  the  shell  and  pass  over  the  viscera, 
including  the  lungs,  and  by  contraction  compress  them 
and  force  out  the  air;  and  partly  (especially  in  land 
tortoises)  by  movements  of  the  shoulder  and  hip  girdles. 
In  most  vertebrates  the  shoulder  girdle  is  movable,  but 
the  hip  girdle  is  fixed;  but  in  ioxioxsts  both  girdles  are 
movable.  In  inspiration  the  shoulder  girdle  is  drawn 
forward  and  the  hip  girdle  backward,  and  the  body 
cavity  is  thus  enlarged.  In  expiration  there  are  con- 
trary movements — i.  e.,  backward  of  the  shoulder  girdle 
and  forward  of  the  hip  girdle,  and  the  body  cavity  is 
contracted.* 

Amphibians. — The  respiration  of  these,  e.  g.,  the 
frog,  is  far  inferior  to  even  that  of  reptiles,  (i)  The 
lung  is  no  sponge  at  all,  but  only  a  sac.  There  is  no 
trachea  and  very  short  bronchi.     The  glottis  opens  from 

*  Charbonnel-Salle,  An.  des.  Sci.  Nat.,  vol.  xv,  art.  6,  1883. 
25 


368   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


the  throat  almost  directly  into  two  large  sacs ;  only 
each  sac  is  sacculated  on  the  interior,  so  as  to  increase 
its  surface  (Fig.  249).  The  aeration  of  the  blood,  there- 
fore, is  still  more  imperfect  than  in  reptiles.  (2)  Again 
amphibians  not  only  have  no  diaphragm,  but  they  have 
no  ribs.  The  mechanics  of  their  breathing  is  entirely 
different  from  all  previously  men- 
tioned. It  is  not  diaphragmatic  nor 
yet  costal.  It  is  a  throat  1  espiration. 
The  submaxillary  space,  or  space  be- 
tween the  branches  of  the  lower  jaw, 
is  large.  This  space  is  pressed  down 
by  the  hyoid  or  tongue  bone,  and 
thus  draws  air  through  the  nostrils 
and  fills  the  throat.  The  nostrils  are 
then  closed,  the  throat  is  brought  up, 
and  the  air  is  forced  down  into  the 
lung  sacs.  This,  of  course,  swells 
the  walls  of  the  abdomen,  which,  re- 
acting by  contraction,  drive  out  the  air  again  through 
the  nostrils. 

This  is  the  lowest  form  of  lung  respiration  and 
grades  into  gill  respiration.  In  fact,  all  amphibians  in 
their  early  larval  life  breathe  by  gills,  and  only  after- 
ward by  lungs.  This,  therefore,  leads  us  naturally  to 
gill  respiration. 


Fig.  249. — One  lung-  of 
a  frog  :  br,  bronchus. 


2.  GILL    RESPIRATION. 

The  most  perfect  form  of  gills,  or  branchicE,  is  found 
in  the  ordinary  typical  fishes  (teleosts).  We  take  this  as 
a  type. 

Take  any  typical  fish,  such  as  a  perch  or  a  salmon 
(Fig.  250).  We  observe  on  each  side  of  the  head  an 
opening  extending  downward  and  forward  to  beneath 
the  chin.     It  is  covered  with  a  movable  flap,  the  oper- 


BLOOD    SYSTEM. 


369 


culum  or  gill  cover.  Lift  this  and  look  in;  we  see  sev- 
eral rows  of  red  gill  fringes,  between  which  there  are 
openings  into  the  throat — gill  slits.  Look  next  into  the 
throat;  we  see  several  cartilaginous  diXQ)c\^%,  gill  arches, 
with  gill  slits  between.  On  these  arches  are  fixed  the 
gill    fringes,  extending    backward    and    outward    under 


Fig.  250. — Head  of  a  fish  with  parts  cut  away  so  as  to  show  the  brain,  spinal 
cord,  and  the  gills  :  cb,  cerebellum  ;  op^  optic  lobes  ;  cr,  cerebrum  ;  olfl, 
olfactory  lobes  ;  olforg,  olfactory  org;an.  rhe  opercle  is  removed  and 
the  gill  fringes  partly  removed,  so  as  to  show  the  four  gill  arches. 

,the  opercle.  The  fringes  are  thin,  flat  plates  fixed  to 
the  gill  arches,  like  the  teeth  of  a  fine  comb  to  its  stem. 
They  are  very  thin,  very  numerous,  and  arranged  in  two 
rows  (Fig.  251).  The  purpose  is  to  make  as  much  sur- 
face as  possible.  They  are  intensely  red,  because  the 
blood  is  profusely  distributed  in  them.  The  blood  passes 
along  each  arch,  and  is  thence  distributed  on  the  fringes. 
Mechanics  of  Breathing. — The  mouth  is  opened 
and  the  throat  enlarged  and  filled  with  water.  By  shut- 
ting the  mouth  and  contracting  the  throat  the  water  is 


70   PHYSIOLOGY    AND    MORPHOLOGY    OF    ANLMALS. 


370 


forced  through  the  gill  slits.  The  current,  quickened 
by  bringing  down  the  opercles,  runs  between  all  the  teeth 
of  the  fringes  and  aerates  the  blood 
there.  This  series  of  movements  is 
continually  repeated. 

The  aeration  of  the  blood  in  gill 
breathers  is  by  means  of  the //r^  oxy- 
gen dissolved  in  the  water.  If  water 
be  boiled  so  as  to  drive  out  all  the 
dissolved  gases,  it  will  no  longer  sup- 
port life.  If  the  gills  of  fishes  be 
kept  moist  and  the  fringes  separated, 
life  out  of  water  may  be  maintained 
for  some  time.  Some  fishes  have  the 
means  of  keeping  their  gills  moist. 
Such  fishes  often  leave  the  water  and 
crawl  about  on  land  for  hours. 

Variation  of  Gills  among 
Fishes. — ^Ve  have  taken  teleosts  as 
a  type.  Gills  of  other  fishes  may  be 
regarded  as  modifications  of  this 
type.  In  sharks  (Fig.  252)  there  are 
five  gill  slits  in  the  throat  and  five 
corresponding  separate  gill  openings 
on  the  sides  of  the  head,  but  not  covered  by  an  opcrcle. 
There  are  cartilaginous  plates  between  the  gill  openings, 
and  the  throat  slits,  and  on  these  are  fixed  the  gill 
fringes.  The  breathing  is  similar  to  that  already  de- 
scribed in  teleosts,  except  in  regard  to  the  movements 
of  the  opercle. 

In  the  lamprey  (petromyzont,  Fig.  253)  we  have 
seven  holes  in  the  throat  and  corresponding  holes  on 
the  side  of  the  neck,  connected  each  by  a  pouch.  In  this 
pouch  are  arranged  the  fringes.  The  breathing  is  the 
same  (Fig.  254). 


Fig.  251. — Transverse 
section  of  a  gill  arch 
GA,  showing  a  pair 
of  fringe  plates  ;  A, 
artery  ;  F,  vein  ;  a, 
arteriole  ;  w,  veinlet. 


BLOOD   SYSTEM. 


In  the  lowest  of  all   fishes,  if  fish  it  may  be  called, 
VIZ.,    the    lancelet    (amphioxus),    the    enormously   large 


Fig.  252. — Anterior  portion  of  a  shark  (Carcharias),  showing;  the  five  gill 
openings. 

throat  has  many — twenty  or  more — slits,  edged  with  im- 
perfect fringes  (Fig.  255). 

Going  up  now  the  other  way,  \n  ganoids,  such  as  the 
garfish  or  bony  pike  [Lepidosteus)  of  our  American  fresh 
waters,  and  in  the  Polyptenis  of  the  Nile,  we  have  an 
opercle  and  gills  like  the  teleosts,  but  gill  breathing  is 


Fig.  253. — Petromyzon  marius,  showing  the  seven  branchial  openings. 
(After  Cuvier. ) 

supplemented  by  a  little  air  breathing  by  means  of  air 
taken  into  a  vascular  air  bladder  (Fig.  256). 

Finally,  in  the  most  reptilian  of  all  fishes,  such  as  the 
protopterus  of  Africa,  the  lepidosiren  of  South  America, 


372    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Jat.o. 


and  the  ceratodus  of 
Australia,  besides  their 
gill  breathing  by  organs 
of  the  pattern  of  the  tele- 
osts,  we  have  nostrils 
opening  into  the  throat 
and  very  good  lungs,  al- 
most as  good  as  some 
amphibians,  and  there- 
fore both  lung  breath- 
ing and  gill  breathing 
in  about  equal  propor- 
tions. These  are  there- 
fore called  /«/7^  fishes. 

Classification  of 
Fishes  by  Respira- 
tory Organs.  —  The 
form  of  the  respiratory 
organs  is  so  characteris- 
tic of  the  various  orders 
of  fishes  that  Huxley 
has  made  it  the  basis  of 
classification.  Accord- 
ing to  Huxley,  fishes 
may  be  divided  into  six 
orders — viz.,  (i)  Dipnoi, 
or  nostril  breathers,  rep- 
resented by  the  protopterus,  lepidosiren,  etc.  These 
breathe  equally  by  lungs  and  gills.  (2)  Ganoids,  so  called 
V      nc 


^-'^\f\\Sj 


Fig.  254. — Anterior  portion  of  a  lamprey 
with  parts  cut  away  so  as  to  show 
the  structure  of  the  gill  pouches  :  mt 
op,  interior  opening  ;  ext  op,  exte- 
rior Opening- ;  r  s,  respiratory  sac. 


Fig.  255. 


-Amphioxus  :  spc,  spinal  cord  ;  nc,  notochord  ;  dv,  dorsal  vessel ; 
st,  stomach  ;  aa,  aortic  arches  or  gill  arches. 


BLOOD   SYSTEM. 


373 


Lei,---  "^'^iyj^"  *.' 


from  their  bony,  enameled  scales,  which  form  a  com 
plete  external  armor.  The}'  supplement  their  gill  breath 
ing  by  a  little  air  breathing. 
They  were  abundant  in  early 
geological  times,  but  are  now 
represented  by  the  Lepidosteus, 
Folypterus,  etc.  (3)  Teieosfs 
(perfect  bone),  or  ordinary 
bony  fishes.  They  breathe 
wholly  by  gills,  but  in  some 
the  air  bladder  opens  into  the 
throat.  (4)  Elas7)iobraiichs 
(plate  gills),  represented  by 
sharks,  skates,  rays,  etc.  (5) 
Marsipobranchs  (pouch  gills), 
represented  by  the  lampreys 
or  conger  eels.  And  (6)  the 
pharyngobranchs  (throat  gills), 
represented  only  by  that  strange  creature  the  amphioxus 
or  lancelet.  The  schedule  will  express  this  classifica- 
tion in  brief  space : 

1.  Dipnoi — lung  fishes. 

2.  Ganoids — armored  fishes. 

3.  Teleosts — typical  fishes. 

4.  Elasmobranchs — sharks,  skates,  etc. 

5.  Marsipobranchs — lampreys. 

6.  Pharyngobranchs — lancelet. 


J  ^^l''  *->' V-'"^:  i 


Fig.  256.— a  portion  of  the  in- 
terior of  the  air  bladder  of  a 
Lepidosteus  i  enlarged  1,  show- 
ing its  cellular  structure. 


TRANSITION    FROM    GILL    BREATHING    TO    LUNG 
BREATHING. 

We  have  seen  pure  gill  breathing  in  teleosts  and 
lower  fishes.  We  saw  pure  lung  breathing  attained  in 
the  higher  amphibians,  as  frogs,  etc.  Now,  in  the  higher 
fishes  and  the  lower  amphibians  we  find  every  gradation 
between — not,  indeed,  by  transformation  of  the  one  into 


374 


PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 


the  other,  but  by  siibstitutioii  of  the  one  for  the  other. 
Some  steps  we  have  already  seen  in  the  higher  fishes, 
the  others  we  find  in  the  lower  amphibians.  In  Gaiioids 
we  saw  a  little  supplementing  of  gill  breathing  by  the 
taking  of  air  into  a  vascular  air  bladder.  This  is  the 
beginning  of  lung  breathing.  In  Dipnoi  wt  saw  an  equal 
gill  breathing  and  lung  breathing.  Now,  in  the  lowest 
amphibians,  such  as  the  siren  and  others,  we  find  but 
little   advance  on  the  Dipnoi.     These   breathe   both  by 

gills  and  by  lungs 
/'^  all      their      lives. 

They  do  not  lose 
their  gills.  They 
are  called  peretmi- 
branchs.  The  nec- 
turus  and  the  siren 
are  examples  of 
these  (Fig.  257). 
In  the  next  higher 
amphibians  the  gill 
breathing  is  confined  to  their  early  life.  They  drop 
their  gills  in  maturity.  These  are  called  cadiicibranchs. 
The  salamanders  and  water  dogs  are  of  this  kind.  In 
the  highest  amphibians,  such  as  frogs  and  toads,  the 
changes  are  greater.  These  lose  their  gills  somewhat 
earlier,  and  also  lose  their  tails,  and  are  therefore  called 
anura,  or  tailless.  Therefore  amphibians  may  be  classi- 
fied thus  : 

Afiura — tailless,  such  as  frogs  and  toads. 

j-T     J  ,       .   -1    1      \  Caducibranchs. 
Urodela — tailed,    -  ,         , 

I  Perenmbranchs. 

Here  there  is  one  great  distinction  between  am- 
phibians and  reptiles.  Amphibians  all  breathe  by  gills 
during  some  period  of  their  lives,  and  some  always; 
reptiles  never. 


Fig.  257. — Head  and  gills  of  Necturus. 


BLOOD   SYSTEM.  3-3 

Again,  fishes  are  all  throat  breathers.  They  must  be, 
for  they  must  force  the  water  outward  through  their 
gills.  Now,  amphibians  also  have  gills,  at  least  in  early 
life,  and  therefore  must  be  throat  breathers,  at  least  in 
early  life.  When  they  become  lung  breathers  they 
retain  the  throat  method,  and  now  force  air  down  to  the 
lungs,  instead  of  water  out  through  the  gills.  Reptiles 
never  have  this  mode  of  breathing. 

Amphibians  were  once  regarded  as  an  order  of 
reptiles,  but  now  they  are  recognized  as  not  only  a 
different  class,  but  as  more  nearly  related  to  fishes  than 
to  reptiles. 

SECTION  III. 
Blood  Ci'rcn/att'on —  /  'ertebrates. 

Having  given  the  general  morphology  of  respira- 
tory organs  among  vertebrates,  we  are  now  prepared 
to  give  the  course  of  circulation,  and  show  how  it  is 
controlled  by  the  necessity  of  aeration.  We  take  first 
mammals,  and,  as  usual,  man  as  the  type. 

Circulation  in  Man. — Circulation  means  a  going 
round  in  a  circle  and  a  coming  back  to  the  starting- 
point.  A  machine  for  circulation  implies  (i)  a  pumping 
organ  and  a  system  of  pipes.  In  the  animal  body  the 
pumping  organ  is  the  heart  and  the  pipes  are  the 
blood  vessels.  It  implies  (2)  two  kinds  of  pipes, 
one  carrying  away,  efferent ;  the  other  bringing  back, 
afferent.  In  the  animal  body  these  are  called  arteries 
and  veins.  There  is,  however,  a  third  kind,  connecting 
these  with  one  another,  called  the  capillaries.  The 
arteries  carry  to  the  tissues,  the  capillaries  among  the 
tissues,  and  the  veins  back  again /;'<?/;/  the  tissues.  The 
arteries  carry  blood  to  the  work,  the  veins  bring  it 
back  after  the  work,  but  the  work  itself  is  done  in  the 
capillaries.     (3)  In  a  good  machine  the  pump  must  have 


3;6   PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

two  chambers — a  receiving  chamber  and  a  pumping 
chamber.  This  latter  chamber  must  be  strong,  for  it 
urges  the  whole  circulation.  In  the  animal  body  the 
receiving  chamber  is  called  the  auricle  and  the  pumping 
chamber  the  ventricle. 

(4)  But  there  are  two  distinct  objects  to  be  subserved 
in  animal  circulation — viz.,  the  nutrition  of  the  tissues 
and  carrying  away  of  waste,  on  the  one  hand,  and  aera- 
tion of  the  blood  on  the  other.  In  the  highest  animals 
these  two  are  wholly  differentiated  and  jealously  kept 
separate.  There  are,  therefore,  two  complete  circula- 
tions and  two  hearts,  each  with  its  two  chambers  (auricle 
and  ventricle)  and  its  three  kinds  of  blood  vessels  (ar- 
teries, veins,  and  capillaries).  But  for  convenience  the 
two  hearts  are  put  together  and  form  a/i^^/r-chambered 
heart,  but  separated  by  a  dead  wall  between.  The  blood 
starts  out  from  the  heart  on  its  journey  in  one  circula- 
tion and  comes  back ;  then  starts  again  on  the  other 
circulation  and  comes  back,  and  so  on  continuously. 
The  first  is  called  the  greater  or  systemic  circulation,  and 
its  function  is  to  nourish  the  tissues  and  carry  away 
their  waste.  The  second  is  the  lesser  ox pulmofiic  circu- 
lation, and  its  function  is  to  aerate  the  blood  or  ex- 
change the  CO2  of  the  blood  for  O  of  the  air.  Of  the  two 
hearts,  that  on  the  left  side  is  the  systemic  and  that  on 
the  right  the  pulmonic.  Through  each  of  these  circula- 
tions the  whole  blood  passes,  and  so  jealously  are  they 
kept  separate  that  although  the  whole  of  the  blood 
passes  through  the  lungs  in  the  pulmonic  circulation, 
yet  none  of  it  is  used  for  the  nutrition  of  the  lungs,  but 
this  work  is  done  by  a  small  branch  from  the  systemic 
circulation. 

General  Course  of  the  Circulation. — Fig.  258 
gives  a  very  generalized  idea  of  this  course.  The  ar- 
rows show  the  course  of  the  current.     Commencing  at 


BLOOD    SYSTEM. 


377 


the  left  ventricle,  the  blood  is  thrown  into  the  great 
arterial  trunk,  called  the  aorta.  This  forms  an  arch — 
the  aortic  arch — from  the  top  of  which  go  branches  to 
the  upper  part  of  the  trunk,  arms, 
and  head,  while  the  main  part 
turns  downward,  running  along 
the  backbone,  branching  and  re- 
branching to  supply  the  viscera 
and  lower  trunk  and  limbs.  It 
left  the  heart  bright  blood  ;  in 
the  tissues  it  gives  up  O  and 
takes  CO2  and  becomes  dark 
blood,  and  in  this  condition  is 
brought  back — that  from  above 
by  the  vena  cava  descendens, 
that  from  below  by  the  vena  cava 
ascendens — to  the  right  auricle. 
The  right  auricle  contracts  and 
throws  it  into  the  right  ventri- 
cle, which  in  turn  throws  it 
through  the  pulmonic  artery,  to 
be  distributed  through  the  capil- 
laries of  the  lungs,  where  it  dis- 
charges its  CO2  and  retakes  O, 
and  becomes  again  bright  blood, 
left  auricle  and  to  the  left  ventricle,  to  commence  again 
another  round. 

Such  is  a  most  generalized  formula.  Fig.  259  sho\.s 
it  still  very  diagrammatically,  but  yet  a  little  more  in 
detail,  especially  as  to  the  great  branches  supplying  the 
liver,  spleen,  the  intestines,  and  the  kidneys,  and  the 
corresponding  veins  to  make  up  the  vena  cava  ascendens. 
Mark  here  the  portal  vein  already  spoken  of  (page  328) 
and  its  singularity. 

In  both  these  diagrams  the  arterial  trunks  are  on  one 


Fig.  258. — Diagp-am  showing 
{general  course  of  the  cir- 
culation :  upeXy  upper  ex- 
tremities and  head  ;  A.v, 
lower  extremities ;  black 
=  dark  blood. 

Thence  it  goes  to  the 


378    PHYSIOLOGY   AND    MORPHOLOGY    OF    ANLMALS. 


Sp 


side  and  the  venous  trunks  on  the  other.  This  is  merely 
for  convenience  of  diagrammatic  representation.  Real- 
ly they  lie  close  together 
along  the  backbone,  as  if 
the  diagram  were  folded 
along  the  middle  backward. 
Again,  of  course,  the  aorta 
in  its  lower  part  divides 
into  two  branches  to  sup- 
ply the  legs,  and  similarly 
the  great  branches  going 
upward  divide  to  supply  the 
arms  and  head.  These  are 
not  represented. 

For  pure  purposes  of 
physiology  the  formula  ex- 
pressed in  these  diagrams 
is  sufficient.  But  there  is 
so  peculiar  a  significance  in 
relation  to  evolution  in  the 
great  oiitgoi/ig  (not  incom- 
tn  ing)  vessels  of  the  heart 
that  a  more  particular  ac- 
count of  these  is  necessary. 
The  aorta  coming  out  of 
the  left  ventricle  (Fig.  260), 
as  already  seen,  makes  an 
arch  to  the  left  and  goes 
down  to  form  the  abdomi- 
nal  aorta.     From   the   top 

Fig.  259. — Diapram  showing^  course      of  the  arch   it  sends  off  up- 
of  circulation,  more  in  detail  :    T  ,     ,  ,  ,  mi 

and  r,  tissues ;  //,  hepatic  artery ;     Ward  three  branches.      1  he 

sp   splenic  artery  ;  m,  mesenteric  ^      ^  is  On    the    right  side  of 
artery  ;  pv,  portal  vem  ;  7\  renal  '^ 

artery  ;  i^,  heart ;  Z,  lungs.    The  the    arch    (left    in    the    dia- 
arrows   show  the   course   of   the  .         rr^,  •  ■       j-     -j 

circulation.    (After  Daiton.)  gram).     This  again  divides 


BLOOD    SYSTEM. 


379 


into  two  branches,  one  (carotid)  going  to  the  right  side 
of  the  head  and  brain,  the  other  (right  subclavian) 
going  to  the  right  arm.  The  ne.xt  in  order  coming  from 
the  arch  is  the  left 
carotid.  It  goes  to 
the  left  side  of  the 
head  and  brain, 
while  the  third  and 
last  bends  to  the 
left  as  the  left  sub- 
clavian and  goes  to 
the  left  arm.  In  the 
right  heart  all  that 
is  necessary  to  p(Mnt 
out  is  the  great  out- 
going vessel.  The 
pulmonary  artery 
arches  to  the  left  to 
go  to  the  left  lung, 
but    sends    back    a 

branch  of  equal  size  to  supply  the  right  lung.  The  sig- 
nificance of  all  of  these  special  arrangements  will  be 
seen  later. 

The  change  from  the  bright  blood  to  the  dark  blood 
takes  place  in  the  capillaries  of  the  tissues  ;  the  change 
back  again  to  bright  blood  in  the  capillaries  of  the 
lungs.  Therefore  in  the  systemic  circulation  the  arteries 
carry  bright  blood  and  the  veins  dark  blood,  while  in 
the  pulmonic  circulation  the  reverse  is  the  case.  All  the 
blood  going  from  the  lungs  to  the  heart  and  from  the 
heart  to  the  tissues  is  bright  blood,  and  all  the  blood 
from  the  tissues  to  the  heart  and  from  the  heart  to  the 
lungs  is  dark  blood.  Or,  all  the  blood  that  ever  visits 
the  left  heart  is  bright  and  all  the  blood  that  ever  visits 
the  right  heart  is  dark.     It  is  common   to  speak  of  the 


Fig.  260. — The  heart  and  the  gjeat  vessels,  in- 
coming and  outgoing^.  The  right  heart  and 
its  vessels  are  shaded. 


38o  PHYSIOLOGY   AND    MORPHOLOGY    OF   ANLMALS. 


bright  blood  as  arterial  and  the  dark  blood  as  venous. 
This  is  true  only  of  the  systemic  circulation.  The  very 
reverse  is  true  of  the  pulmonic  circulation. 

Structure  of  the  Heart :  The  Valves.— We  have 

given  the  course  of  the  circulation  ;  but  the  question 
arises,  How  is  it  maintained  continuously  in  the  same 
direction  ?  This  is  done  in  the  same  way  as  it  is  done 
in  machinery — viz.,  by  a  system  of  valves  v^\i\c\\  prevents 
its  going  in  the  other  direction. 

The  valves  of  the  heart  are  of  two  kinds,  which  may 
be  called  curtain  valves  and  pocket  valves  (semilunar 
valves).     The  former  are  between  the  two  chambers  of 

one  heart,  to  prevent 
py^  ^^^^^^  the  blood  from  going 

back  to  the  auricle; 
the  latter  are  placed 
at  the  outlet  of  the 
ventricles  into  the 
great  outgoing  arter- 
ies, i.  e.,  at  the  base 
of  the  aorta  and  the 
pulmonary  artery  to 
prevent  the  blood  dis- 
charged into  these  ar- 
teries from  falling 
back  into  the  heart 
when  the  ventricle  ex- 
pands again.  The  cur- 
tain valves  I  so  call  because  they  are  thin  membranous 
curtains  between  the  two  chambers  of  the  heart  and 
opening  always  into  the  ventricle.  From  the  interior  of 
the  wall  of  the  ventricle  there  go  cords  (heart  strings), 
which  are  attached  to  the  edges  of  the  curtain,  so  that 
while  these  open  easily  into  the  ventricle,  allowing  blood 
to  pass  (Fig.  261,  A),  yet  if  blood  attempts  to  pass  back 


Fig.  261. — Left  heart.  A,  auricle  contracted, 
ventricle  expanded.  B,  ventricle  con- 
tracted, auricle  expanded :  /z',  pulmo- 
nary vein  ;  aii,  auricle  ;  z',  ventricle  ;  ao, 
aorta. 


BLOOD   SYSTEM. 


381 


Fig.  262. — Aorta  cut  open 
and  spread  out  to  show 
the  semilunar  valves. 


into  the  auricle  they  flap  back,  pressing  against  one  an- 
other, closing  the  way,  and  are  held  in  place  by  the 
cords*  (Fig.  261,  B).  The  valve  between  the  right 
auricle  and  ventricle  is  called  the  tricuspid,  because  it 
has  three  cusps.  That  in  the  left 
heart  is  called  the  bicuspid,  as  hav- 
ing only  two  cusps. 

The  semilunar,  or  pocket  valves 
are  at  the  base  of  the  aorta  and 
of  the  pulmonary  artery,  three  in 
each,  of  crescentic  shape  like  shal- 
low pockets,  which,  when  filled, 
press  against  each  other  from 
three  sides  and  completely  close 
the  artery.  When  the  ventricle 
contracts,  the  blood  rushes  into 
the  artery,  the  valves  pressing 
close    against    the    wall;    but   as 

soon   as   the    ventricle    relaxes,   the   pockets    fill,    press 
against  each  other,  and  close  the  gate  (Fig.  262). 

We  now  follow  again  the  course  of  the  blood,  show- 
ing how  the  valves  work.  Blood,  dark  from  the  tissues, 
enters  the  right  auricle.  The  auricle  contracts,  the  cur- 
tains flap  wide  open  into  the  ventricle,  and  the  blood 
enters  freely.  The  ventricle  contracts,  the  blood  shuts 
the  curtains,  they  are  held  in  place  by  the  cords,  and 
the  blood  is  forced  into  the  pulmonic  artery.  When  the 
ventricle  relaxes,  in  order  to  fill  itself  again,  the  pocket 
valves  fill  and  close  the  way.  The  action  on  the  other 
side  of  the  heart  is  the  same.  The  blood  coming  from 
the  lungs  into  the  auricle  is  thrown  into  the  ventricle  and 
the  door  closed  behind.  It  is  therefore  forced  upward 
into  the  aorta  and  again  the  door  closed  behind.     It  is 

*  To  appreciate  the  action  of  these  they  must  be  examined  on 
a  heart  or  a  good  model. 


382    PHYSIOLOGY   AND    MORPHOLOGY    OF  ANIMALS. 

the  flapping  and  closing  of  these  valves,  both  curtain 
and  pocket,  that  make  the  characteristic  sounds  of  the 
beating  heart,  and  it  is  the  inflammation  and  thickening 
of  them  that  constitute  the  commonest  and  gravest  dis- 
ease of  the  heart. 

Blood  Vessels. —  i.  Arteries. — In  the  dissection  of 
a  body  the  arteries  can  be  at  once  distinguished  from 
the  veins  by  the  fact  that  they  retain  their  form  as  pipes, 
while  the  thinner  veins  collapse. 

Structure. — They  consist  of  three  coats  :  an  outer  thin, 
very  tough  fibrous  coat;  a  middle  elastic,  muscular,  or 
proper  coat ;  and  an  inner  lining  epithelial  coat.  The 
outer  coat  gives  toughness,  the  middle  gives  elasticity 
and  firmness,  the  inner  is  in  some  way  necessary  to  the 
life  and  integrity  of  the  blood.  In  tying  an  artery  the 
surgeon  draws  the  thread  until  he  feels  the  cutting  of 
the  middle  and  interior  coats,  and  then  secures  the  knot. 
In  aneurism  the  inner  and  middle  coats  are  broken 
and  the  tough  outer  coat  is  extended  into  a  pulsating 
tumor.  The  firmness  of  the  middle  coat  is  necessary 
to  maintain  an  open  passage  for  the  blood  under  power- 
ful action  of  the  heart,  and  by  its  elasticity  it  yields  to 
the  impulse  of  the  heart,  and  then,  contracting,  it  carries 
forward  the  blood  in  its  course.  It  is  this  propagated 
wave  of  swelling  and  contraction  that  constitutes  the 
pulse.  From  the  great  aortic  trunk,  of  course,  the 
arteries  branch  and  rebranch  until  they  become  of  capil- 
lary fineness  in  all  the  tissues,  and  finally  grade  into  the 
capillaries  proper.  The  blood  does  not  ooze  through 
the  arterial  walls  ;  these  pipes  do  not  leak.  The  nutri- 
tion of  the  tissues  is  the  function  of  the  capillaries  alone. 

2.  Veins. — The  veins  are  much  larger  than  the  arteries, 
and  yet  far  less  conspicuous,  because  they  collapse  when 
empty.  In  structure  they  are  similar  to  the  arteries. 
They,  too,  have  three  coats,  but  the  middle  coat  is  much 


BLOOD    SYSTEM. 


383 


thinner.  The  veins  are  found  in  two  positions;  (1) 
deep-seated  and  accompanying  the  arteries,  and  (2) 
superficial  or  subcutaneous.  It  is  the  subcutaneous 
which  show  bluish  through  the  skin,  especially  in 
blondes.  The  greater  size  and  number  of  the  veins  is 
necessary  because  the  blood  current  is  sluggish  in  them 
as  compared  with  the  arteries. 

Valves. — The  comparative  sluggishness  of  the  current 
is  also  the  reason  for  the  existence  of  semilunar  valves 
in  veins.  These  are  not  in  triplets,  as  at  the  opening 
of  the  great  arterial  trunks  into  the  heart.     They  are 


B 


Fig.  263. — Vein  :  A,  cut  open  so  as 
to  show  the  valves,  v  ;  B,  section 
through  the  valves. 


Fig.  264. — a,  arteriole  ;  v,  veinlet ; 
Cy  capillary  network.  The  ar- 
rows show  the  course. 


scattered  along  their  course  irregularly  and  singly. 
They  can  not,  therefore,  arrest,  but  only  retard  the 
backward  flow  of  the  blood.  The  knotted  appearance 
of  the  subcutaneous  veins  when  gorged  with  blood  (as 
when  the  arm  is  corded)  is  due  to  the  filling  of  these 
valves  (Fig.  263). 

3.   Capillaries. — The  blood  system  is  a  closed  system  of 
pipes.     There  is  no  discharge  of    the    blood   from  the 
arteries  into  the  tissues  and   taking  up  of  it  from  the 
26 


384    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

tissues  by  the  veins ;  on  the  contrary,  the  arterial 
pipes  are  continuous  through  the  capillaries  with  the 
veins.  Yet  the  capillaries  are  quite  distinct  from  both 
in  several  respects:  i.  In  the  mode  of  branching.  The 
arteries  branch  and  rebranch  tissneward,  growing  ever 
smaller  and  more  numerous;  the  veins  run  together 
heartivard,  growing  ever  larger  and  fewer  ;  the  capil- 
laries branch  and  run  together  again,  without  change  of 
size  and  without  definite  direction,  forming  a  fine  «<?/- 
work  (Fig.  264).  2.  ///  the  course  of  the  current.  The 
course  of  the  current  in  arteries  and  in  veins  are  alike 
fixed.  In  capillaries  it  may  go  in  any  direction.,  although 
it  works  deviously  toward  the  veins.  3.  hi  structure. 
The  walls  of  the  capillaries  are  of  extreme  thinness,  and 
therefore  permeable  to  the  blood  plasm,  and  in  case  of 
engorgement  even  to  the  leucocytes.  4.  In  function. 
The  function  of  the  arteries  is  to  carry  the  blood,  without 
loss  by  leakage,  to  the  tissues;  the  function  of  the  veins  is 
to  bring  it  back  without  loss  to  the  heart ;  the  function 
of  the  capillaries  is  to  allow  the  blood  to  exude  into  the 
tissues.  In  any  good  system  of  irrigation  the  pipes  car- 
rying the  water  to  the  soil  and  bringing  it  back  from 
the  soil  must  be  /w/^rzw/zi',  but  those  running  among 
the  soil  must  be  permeable.  The  blood  system  is  such 
a  system  of  pipes.  So  jealously  is  this  function  of 
nutrition  restricted  to  the  capillaries  that  even  the 
arteries  and  veins  themselves  are  not  nourished  by  the 
blood  that  flows  through  them,  but  must  have  their  own 
arterioles,  veinlets,  and  capillaries  (vasa  vasorum)  for 
that  purpose.  In  the  capillaries,  then,  the  nutritive 
matters  of  the  plasma  and  the  oxygen  of  the  red  glob- 
ules exude  into  the  tissues  (exosmose),  and  the  waste 
matters  of  the  tissues  and  the  COg  transude  into  the 
blood  (endosmose).  But  the  exact  nature  of  this  pro- 
cess is  yet  obscure. 


BLOOD    SYSTEM.  385 

COMPARATIVE     MORPHOLOGY    OF    THE    BLOOD     SYSTEM     IN 
VERTEBRATES. 

Mammals. — In  all  essential  features  the  blood  sys- 
tem of  mammals  is  exactly  the  same  as  that  of  man, 
already  described.  All  mammals  have  a  four-chambered 
heart — i.  e.,  a  double  heart  with  no  communication  be- 
tween except  through  the  capillaries.  They  all,  there- 
fore, have  a  complete  double  circulation,  with  the  whole 
blood  passing  through  both. 

Birds. — Birds  have  also  a  double  heart  and  complete 
double  circulation.  There  is  only  one  thing  worthy  of 
note,  and  that  only  on  account  of  its  significance  in  the 
evolution  of  birds.  It  is  that  the  aortic  arch  turns  to 
the  right  instead  of  to  the  left,  as  in  mammals. 

Reptiles  and  Amphibians. — The  first  important 
variation  from  the  model  already  given  is  found  in  these. 
The  heart  of  the  reptile  and  the  amphibian  is  three- 
chambered  instead  of  four-chambered.  If  the  heart  of 
mammals  and  birds  may  be  compared  to  two  tenements, 
each  with  two  chambers,  joined  together,  but  with  dead 
wall  between,  then  the  heart  of  reptiles  and  amphibians 
may  be  compared  to  one  tenement  with  a  suite  of  three 
rooms.  With  such  a  structure  it  is  impossible  to  have  a 
complete  double  circulation.  The  pulmonic  circulation 
is  only  a  branch  of  the  systemic,  and  therefore  only  a 
part  of  the  blood  passes  through  the  lungs  to  be 
aerated,  and  therefore  also  not  pure  oxygenated,  but 
more  or  less  mixed  blood  goes  to  the  tissues. 

Course  of  Circulation. — Fig.  265  is  a  schematic 
diagram  showing  the  course  of  circulation  in  reptiles. 
The  heart,  as  is  seen,  consists  of  two  auricles  and  one 
ventricle.  The  blood  is  thrown  out  of  the  ventricle  into 
the  aorta,  which  makes  an  arch,  but  there  divides  and 
sends  a  large  branch  to  the  lungs,  while  the  main  branch 


386 


PHYSIOLOGY    AND    MORPHOLOGY   OF   ANLMALS. 


goes  to  the  tissues  above  and  below.  That  which  goes 
to  the  tissues  comes  back  to  the  right  auricle  as  dark, 
and  that  which  goes  to  the  lungs  to  the  left  auricle  as 
bright  blood.  The  auricles  con- 
tract and  empty  their  contents 
into  the  common  ventricle  as 
mixed  blood,  which  is  again 
thrown  into  the  aorta,  to  be 
again  divided  between  the  tis- 
sues and  the  lungs. 

There  is  a  double  reason  why 
these  animals  are  cold-blooded : 
(i)  We  have  already  seen  that 
the  lungs  in  them  are  a  coarse 
sponge,  and  therefore  the  sur- 
face of  exposure  of  the  blood 
to  the  air  is  small ;  (2)  and  now 
we  see  that  not  the  whole  but 
only  a  part  of  the  blood  is  oxi- 
dized in  each  round  of  the  cir- 
culation. Mixed  blood  goes  to 
the  tissues.  The  metabolic  process — i.  e.,  waste  and 
supply — is  less  active,  and  therefore  the  heat  is  less. 

We  have  given  in  Fig.  265  a  schematic  diagram  illus- 
trating the  general  principle.  This  is  sufficient  for 
physiology  but  not  for  morphology,  and  especially  not 
for  evolution.  The  actual  course  is  far  more  complex, 
and  it  differs  also  a  little  in  different  reptiles.  We  can 
not  take  all  the  cases.  We  take  the  typical  case  of  the 
lizard. 

The  lizard  (Fig.  266)  has  three  aortic  arches  on  each 
side — six  in  all.  What  conceivable  use  can  there  be  for 
six  aortic  aches  ?  The  blood  from  two  of  these — the 
lower  one  on  each  side — goes  to  the  lungs  to  be  aerated, 
while  that  of  the  other  four,  after  sending  a  branch  to 


Fig.  265. — Diagram  showing 
the  structure  of  the  heart 
and  the  general  course  of 
the  circulation  in  reptiles 
and  amphibians. 


BLOOD   SYSTEM. 


38/ 


the  head,  unite  to  form  the  abdominal  aorta  which  goes 
to  supply  the  viscera  and  lower  portion  of  the  trunk  and 
limbs. 

I  have  taken  the  lizard  as  a  type  both  of  the  three- 
chambered  heart  and  of  the  structure  of  the  aortic  arches. 
But  there  is  considerable  variation  among  reptiles  in 
both  of  these,  and  these 
variations  show  transitions 
such  as  one  would  expect 
on  the  theory  of  derivative 
origin  of  organic  forms. 
For  example,  the  perfect 
three-chambered  heart  is 
general  among  reptiles,  as 
in  lizards  and  tortoises, 
and  also  in  all  amphibians 
— e.  g.,  in  frogs,  toads,  etc. ; 
but  in  serpents  there  is  an 
imperfect  four-chambered 
heart.  The  ventricle  has 
been  partly  but  not  com- 
pletely divided.  In  the 
crocodile  there  is  a  cotn- 
plete  four-chambered  heart, 
but  still  the  blood  is  mixed 
in  the  course  of  the  circu- 
lation, though  not  in  the 
heart.  Also  the  arches  are 
more  or  less  modified  from 
the  type  given  above. 

Fishes. — Fishes  have 
chambered  heart,  and  yet  the  whole  of  the  blood  passes 
through  the  gills  to  be  aerated,  and  therefore  pure  blood 
only  goes  to  the  tissues.  Fig.  267  is  a  schematic  dia- 
gram showing  the  general  course  of  the  circulation  in  a 


Fig,  266. — Showing  heart  and  out- 
going blood  vessels  of  a  lizard. 
The  arrows  show  the  course  of 
the  blood.     (After  Owen.) 


a    Still  simpler,  viz.,  a   t7vo- 


388    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


teleost  fish.  The  blood  from  the  single  ventricle,  v  (dark 
blood),  is  thrown  into  the  aorta,  which  then  divides  into 
three  or  four  arches  on  each  side,  in  order  to  pass  through 
the  gill  arches,  to  be  distributed 
in  the  gill  fringes  and  aerated 
there.  From  the  gill  fringes  it 
is  gathered  as  bright  blood,  and 
passes  on  to  unite  into  the  ab- 
dominal aorta  ivithout  going  back 
to  the  heart,  and  is  then  distrib- 
uted to  the  tissues.  Its  course 
is  shown  by  the  arrows. 

I'his  is  schematic.  The  ac- 
tual course  is  shown  in  Fig.  268, 
which  shows  also  the  change  in 
the  blood  in  passing  through 
the  gill  fringes.  As  to  variation 
from  this  type,  it  is  sufficient  to 
say  that  in  sharks  there  are  five 
arches,  in  lampreys  seven,  and 
in  lancelets  tw-enty  or  more  on 
each  side.  On  the  other  hand, 
the  Dipnoi  or  lung  fishes,  lepido- 
siren,  and  protopterus,  like  the  reptiles,  have  only  three 
on  each  side. 

Several  interesting  questions  occur  here,  (i)  We 
have  seen  that  fishes  have  a  single  heart.  Which  is  it  ? 
Physiologically  it  is  a  pulmonic  heart,  for  nothing  but 
dark  blood  visits  it.  But  morphologically  it  doubtless 
corresponds  to  the  whole  double  heart  of  mammals.  (2) 
Is  the  circulation  of  reptiles  or  that  of  fishes  the  better? 
Physiologically,  in  some  respects  the  one,  in  some  re- 
spects the  other  is  the  better.  The  reptile  has  the  ad- 
vantage of  air  breathing,  which  aerates  the  blood  more 
efficiently  than  water  breathing,  but,  on  the  other  hand, 


Fig.  267. — Diagram  showing 
structure  of  the  heart  and 
course  of  circulation  in 
fishes. 


BLOOD    SYSTEM. 


389 


the  fish  has  the  advantage  of  aerating  the  whole  of  the 
blood,  and  therefore  sending  only  aerated  blood  to  the 
tissues,  while  in  reptiles  only  a  part  of  the  blood  is 
aerated,  and  therefore  mixed  blood  goes  to  the  tissues. 
Per  contra,  however,  the  fish  has  again  the  disadvan- 
tage of  the  blood  not  coming  back  from  the  gills  to  the 
heart  to  receive  fresh  impulse,  so  that  the  heart  has  to 
do  the  double  duty  of  sending  the  blood  by  one  impulse 


Fig.  268. — A,  heart  and  gill  arches  of  a  fish  ;  B,  one  arch  with  fringe  (after 
Owen) ;  //,  the  heart.     Dark  blood  is  shaded. 


through  two  capillary  circulations.  In  a  morphological 
point  of  view  the  reptile  is  undoubtedly  the  higher  form 
of  circulation,  for  it  is  a  transition  to  the  still  higher 
forms.  The  three-chambered  heart  is  a  transition  from 
the  two-chambered  heart  of  the  fish  to  the  four-cham- 
bered heart  of  birds  and  mammals,  and  what  might  be 
called  the  one-and-one-half  circulation  of  reptiles  a  tran- 
sition from  the  single  circulation  of  the  fish  to  the  double 
circulation  of  the  bird  and  mammal. 


390 


PHYSIOLOGY   Ax\D    MORPHOLOGY    OF    ANLMALS. 


BEARING     OF    SOME    OF    THESE    FACTS    ON    EVOLUTION. 

1.  Heart  Structure. — There  is  abundant  evidence, 
both  in  the  taxouomic  series  (animal  scale)  and  the 
embryonic  series,  that  the  heart  has  been  gradually 
formed  by  a  process  of  evolution,  for  nearly  all  the 
stages  may  be  traced  from  the  simplest  pulsating  organ 
to  the  complex  four-chambered  valvulated  structure  of 
the  higher  animals.     The  steps  are  briefly  as  follows: 

(i)  First  there  is  a  simple  pulsating  dorsal  vessel.  This 
stage  is  found  in  many  invertebrates,  and  even  among 
vertebrates  in  the  amphioxus. 

(2)  Then  comes  a  one-chambered  heart.  This  is 
found  in  many  invertebrates. 

(3)  Then  comes  the  two-chambered  heart  of  many 
invertebrates,  and  of  fishes  among  vertebrates. 

(4)  Then  the  three-chambered  heart  of  the  typical 
reptile  and  amphibian. 

(5)  Then  the  imperfect  four-chambered  heart  of  ser- 
pents, the  two  sides  of  the  heart  still  connected. 

(6)  Then  the  perfect  four-chambered  heart  of  croco- 
dilians  ;  but  even  yet  a  connection  (ductus  arteriosus)  by 
which  the  blood  to  the  tissues  is  mixed,  and  therefore 
the  circulation  is  not  completely  double. 

(7)  The  final  step  is  the  perfect  four-chambered  heart 
and  complete  double  circulation  of  birds  and  mammals. 

Nearly  all  these  steps  are  found  also  in  the  embry- 
onic development  of  mammals  and  even  of  man. 

2.  Origin  of  Aortic  Arches. — But  by  far  the  most 
interesting  question  in  this  regard  is  the  origin  and 
meaning  of  aortic  arches. 

We  have  already  drawn  attention  to  the  fact  that  the 
aortic  arch  in  the  bird  turns  to  the  right,  and  not  to  the 
left,  as  in  mammals.  This  shows  that  mammals  did  not 
come  from  birds  by  modification,  for  the  intermediate 


BLUOD    SYSTEM. 


391 


Stages  from  a  right-turning  to  a  left-turning  arch  would 
be  unsuitable.  We  will  show  that  they  both  came  from 
reptiles,  but  by  different  routes. 

Again  we  have  drawn  attention  to  the  strange  fact 
that  in  reptiles  (e.  g.,  the  lizard)  there  are  three  arches 
on  each  side — six  in  all.  Why  six  arches  ?  Surely,  one 
arch  IS  enough :  for  birds  and  mammals  have  but  one, 
and  they  have  the  most  perfect  circulation  of  all.  The 
key  to  the  mystery  is  found  in  the  circulation  of  fishes. 
Fishes  have  three  or  four  or  five  arches  on  each  side,  but 
the  reason  in  their  case  is  obvious.  They  are  the  gill 
arches.  They  are  absolutely  necessary  for  the  aeration 
of  the  blood.  If  the  reptiles  came  by  modification  from 
fishes,  then  the  explanation  of  their  numerous  arches  is 
plain.  They  are  the  rcvmants  of  ancestral  gill  arches. 
Some  of  the  highest  of  fishes,  the  Dipnoi,  have  only 
three  arches  on  each  side  like  the  lizard.  These  fishes 
also  breathe  by  lungs  as  well  as  by  gills.  Now,  sup- 
pose in  successive  generations  the  lungs  of  Dipnoi  to 
increase  in  efficiency,  and  the  gills  to  dwindle  and  dry 
up,  it  is  evident  that  exactly  the  structure  found  in  the 
lizard  would  remain. 

That  exactly  this  did  take  place  in  the  history  of  the 
evolution  of  vertebrates  is  proved  by  the  fact  that  every 
step  is  found  now  among  fishes,  amphibians,  and  reptiles, 
and  furthermore  that  the  same  change  takes  place  now 
in  the  individual  history  of  every  one  of  the  higher  am- 
phibians, as,  for  example,  the  frog.  We  have  already 
traced  the  successive  steps,  through  teleosts,  ganoids, 
Dipnoi,  perennibranchiate  amphibians,  caducibranchiate 
amphibians,  to  reptiles.  We  wish  now  to  trace  the  same 
change  in  the  individual  development  of  a  frog. 

The  frog,  as  we  all  know,  is  at  first  a  tadpole  ;  without 
feet,  swimming  only  by  the  tail;  without  lungs  or  air 
breathing — in  fact,  breathing  only  water  by  gills.     In  this 


392 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


state  it  is  essentially  a  fish.     Now  this  fishlike  gill-breath- 
ing animal  changes  into  a  lung-breathing  land  animal, 


Fig.  270. 

Figs.  269,  270. — Diagrams  showing  the  change  of  the  course  of  blood  in  the 
development  of  a  frog.  Fig.  269,  the  tadpole  stacre.  Fig.  270,  the  ma- 
ture condition:  //,  heart;  G  G'  G  \  external  gills;  ff  g' g'  ,  internal 
gills  ;  c  c,  connecting  branches  in  the  tadpole  ;  //,  pulmonary  branches. 

and  in  doing  so  the  gill  arches  are  changed  into  aortic 
arches  before  our  eyes.     The  process  is   a   little  more 


BLOOD    SYSTEM. 


393 


1 


^^^/^ 


complex  than  I  have  represented,  because  the  amphibian 
has  external  as  well  as  internal  gills.  But  the  same  was 
doubtless  true  in  the  evolution  of  reptiles.  How,  then, 
does  the  change  take  place  ?  Fig.  269  represents  the  cir- 
culation of  the  embryo 
or  tadpole,  and  Fig.  270 
the  adult.  In  the  embryo 
(Fig.  269),  although  all 
the  blood  goes  through 
the  gills,  yet  note  rudi- 
mentary vessels  to  the 
rudimentary  lungs,  not 
yet  used,  and  also  rudi- 
mentary vessels  connect- 
ing gill  arch  to  gill  arch. 
Again  note  in  the  adult 
(Fig.  270)  remnants  of 
dried-up  gill  vessels.  The 
letters  are  the  same  in 
the  two  figures,  and  the 
arrows  show  the  direc- 
tion of  the  current. 

We  have  thus  proved     Fig.    271.— ideal  diagram    representing 

the  origin  of  the  aortic  rIXT''""  ^''''''  "'*'''•    ^^^''' 

arches     of     the     lizard. 

Now,  the  same  is  true  of  all  aortic  arches,  even   those  of 
man  himself. 

Birds  and  Mammals. — In  birds  and  mammals  the 
modification  has  become  so  great  as  to  obscure  the 
homology.  The  proposition  to  be  proved  is  that  the 
great  outgoing  vessels  of  the  heart  are  the  modified  rem- 
nants of  gill  arches.  Indeed,  this  is  obvious  enough  if 
we  take  the  early  embryonic  condition  of  a  bird  or 
mammal.  The  early  mammalian,  and  even  human, 
embryo  has  gill  slits,  several  of  them  on  each   side  of 


394 


PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 


the    neck,    and    gill    arches    through    which    the    blood 
passes,  forming,  in   fact,  several  aortic    arches  on  each 

side.  But  afterward 
these  are  modified  into 
the  outgoing  vessels 
of  the  heart,  and  their 
original  arch  form  is 
retained  only  by  one. 
The  manner  in  which 
the  change  takes  place 
is  shown  in  Figs.  271, 
272,  and  273.  Fig.  271 
shows  in  a  schematic 
way  the  primitive  aor- 
tic or  gill  arches  (al- 
({,  most  exactly  repre- 
sented now  by  sharks), 
and  Figs.  272  and  273 
the  same  as  it  is  modi- 
fied for  a  bird  and 
mammal  respectively. 
Finally,  Fig.  274  is  the 
mammalian  heart  slightly  modified  to  suggest  the  homol- 
ogy of  the  several  parts. 

To  explain:  The  gill  arches  are  five  in  number  on 
each  side,  as  in  a  shark  (Fig.  271).  The  two  upper  pairs 
are  soon  aborted,  even  before  leaving  the  class  of  fishes, 
and  only  three  on  each  side  remain  to  be  accounted  for. 
These  are  the  three  found  in  the  lizard,  and  inherited 
with  modifications  in  birds  and  mammals.  These  are 
therefore  the  only  ones  we  have  to  deal  with.  Fig.  272 
shows  the  modification  of  this  formula  in  birds.  It  is 
seen  that  the  first  arch  on  each  side  becomes  the  two 
pulmonic  arteries,  one  going  to  each  lung,  precisely  as  in 
the  lizard  and  in   the  frog.     Of   the  next  pair  of  arches, 


Fig.  272. — Modified  for  bird. 


BLOOD   SYSTEM. 


395 


that  on  the  right  (left  of  the  figure)  becomes  the  aorta, 
and  that  on  the  left  the  subclavian  on  that  side.  The 
subclavian  on  the  other  side  is  a  branch  of  the  aorta. 
The  next  pair  became 
the  carotids.  They  are 
already  so  in  the  lizard 
and  in  the  frog  (see  Fig. 
270). 

In  the  mammals  the 
modification  is  a  little 
different.  It  is  seen 
(Fig.  273)  that  of  the 
^rst  pa.ir  of  arches,  that 
on  the  left  is  the  pul- 
monary artery,  which  in 
this  case  supplies  both 
lungs,  while  that  on  the 
right  is  aborted.  Of  the 
second  pair,  that  on  the 
left  (right  of  the  figure) 
becomes  the  aorta,  while 
that  on  the  right  the 
subclavian  on  that  side.  The  third  pair,  as  in  birds,  and 
as  already  in  reptiles  and  amphibians,  becomes  the  carot- 
ids. In  Fig.  274  we  give  a  figure  of  the  heart,  with  its 
outgoing  vessels  a  little  modified,  to  suggest  their  ho- 
mology, and  numbered  so  as  to  show  the  corresponding 
parts. 

We  see  now  that  the  only  difference  between  bird 
and  mammal  in  the  aorta  and  other  outgoing  vessels  of 
the  heart  is  that  among  the  various  arches  different  ones 
have  been  selected  to  be  retained  in  the  arch  form  as 
aorta  and  for  pulmonary  artery  and  for  subclavian. 
In  both  cases — i.  e.,  in  both  birds  and  mammals — the 
steps  of  the  change  may  be  traced  in  the  embryo. 


Fig.  273. — Modified  for  mammal. 


396   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Illustration  of  a  Fwidatnental  Law  of  Evolution. — It  is  a 
fundamental  law  of  evolution  that  t\\Q  phylogenic  or  evo- 
lution series  is  repeated  more  or  less  perfectly  in  the 
ontogenic  or  embryonic  series,  and  often  also  in  the  taxo- 
iwviic  or  classification  series.  An  admirable  illustration 
of  this  IS  found  in  the  life  history  of  amphibians. 

We  have  already  seen  that  the  frog  in  its  embryonic 
development  is  at  first  a  legless,  gill-breathing  animal. 


Fig.  274. — Diagram  of  mammalian  heart:  a,  aorta;  /,  pulmonary  artery; 
jc,  s  c\  subclavium  on  each  side  ;  c,  c\  carotids  on  each  side.  The 
numerals  give  the  ordinal  numbers  of  the  arches  as  in  the  previous  figures. 

If  it  stopped  there  it  would  be  classed  as  a  fish  ;  but  it 
goes  on.  It  next  gets  itself  legs  and  an  imperfect  lung, 
and  breathes  now  both  air  and  water.  If  it  stopped  here 
it  would  be  classed  as  a  perennibranch  ;  but  it  goes  on. 
It  next  loses  its  gills  and  improves  its  lungs  and  breathes 
air  only,  but  retains  the  tail.     If  it  stopped  now  it  would 


BLOOD   SYSTEM. 


397 


be  classed  as  a  caducibranch ;  but  it  still  presses  on. 
Finally,  it  loses  its  tail  and  reaches  the  highest  order  of 
amphibia — the  Anura.  Of  course  the  caducibranchs 
and  perennibranchs  pass  through  similar  stages,  but 
stop  earlier.  So  much  for  the  similarity  of  the  onto- 
genic  and  taxonomic  series. 

Now,  there  can  be  no  doubt  that  the  phylogenic  series 
is  also  similar,  that  the  amphibians  were  evolved  from 
the  dipnoan  fishes,  and  that  in  the  course  of  geologic 
times  they  passed  through  the  same  stages — i.  e.,  peren- 
nibranch,  caducibranch,  and  became  anurous  only  in 
Tertiary  times.  Thus  the  three  series  are  similar.  All 
these  facts  are  expressed  in  the  following  diagram : 


Anura  — ►  = 
Caducibranch - 
Pereaaihrancb ■ 
Fish 


Ontogenic 


-*Fish~^Perennibrancb^^c:aducibraBcb  ->■  Anura ' 
■i-  Ss  3  4 


-Fisb—*  Perennibrancb 


Fig.  275. 


SECTION    IV. 

Morphology  of  Respiratory  and  Circulatory  System  in 
Invertebrates. 

Some  General   Introductory  Remarks. — (1)  In 

the  case  of  vertebrates  we  gave  first  the  morphology  of 
the  respiratory  organs  and  then  of  the  circulatory  organs 
separately,  but  in  the  case  of  the  invertebrates  we  shall 
take  these  together  in  each  class  treated. 

(2)  In  the  vertebrates  the  blood  system  is  a  system 
of  closed  pipes,  continuous  and  without  either  discharge 
or  intake,  e.xcept  by  exosmose  and  endosmose.    In  nearly 


398 


PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 


all  invertebrates  there  are  blood-sinuses  or  reservoirs 
in  various  parts  of  the  body,  into  which  the  blood  is 
discharged,  and  from  which  it  is  again  taken  up. 

(3)  In  vertebrates  the  oxidized  blood  is  bright  red 
and    the    deoxidized    blood  dark    purple-red,  and    this, 

therefore,  is  the  conven- 
tional mode  of  repre- 
senting these  two  kinds 
of  blood  in  colored  dia- 
gram. In  default  of  col- 
or we  have  used  unshad- 
ed and  shaded  spaces  to 
represent  the  same.  We 
continue  to  use  the  same 
purely  conventionally, 
although  in  the  case  of 
invertebrates  there  is  no 
such  marked  change  in 
color  or  shade. 

(4)  The  diversity 
among  invertebrates  is 
so  extreme  that  it  is  im- 
possible to  do  more  than 
select  a  few  striking  ex- 
amples from  each  de- 
partment. 

(5)  In  this  selection 
we  pass  over  insects  for 
the  present  to  come  back 
to  them.     The  reason  is 


Fig.  276. — Lobster  with  the  carapace 
taken  off :  st,  stomach  ;  dotted  tube, 
it,  intestines;  Z,  liver;  //,  heart; 
G,  gills  ;   GG,  green  gland. 


the  same  which  induced  us  to  do  so  in  giving  the  com- 
parative morphology  of  the  eye  (page  162) — viz.,  that  in 
respiration  and  circulation  insects  are  entirely  peculiar, 
and  are  outside  of  the  direct  line  of  evolution,  or  of 
increasing  complication  as  we  go  up. 


BLOOD   SYSTEM. 


399 


Arthropods :  Crustacea. — As  an  example  of  the 
department  of  arthropods,  therefore,  we  select  crusta- 
ceans. 

Respiratory  Organs. —  Take  a  crab,  lobster,  or  crawfish, 
and  remove  the  carapace  or  dorsal  shell.  Directly 
exposed  on  each  side  and  occupying  a  large  part  of  the 
upper  surface  are  seen 


a  great  number  of  taper- 
ing, finely  lamellated  or 
tufted  organs,  Fig.  276, 
G  G.  These  are  the 
gills.  They  are  not  7vith- 
in  the  body  cavity,  but 
wholly  outside.,  in  special 
respiratory  chambers, 
opening  by  a  large  cleft 
on  each  side  of  the  shell 
(Fig.  277).  The  gills 
are,  some  of  them,  con- 
nected each  with  a  limb 
or  a  maxilliped,  and  are 
indeed  appendages  of 
these,  and  some  with  the 
thoracic  walls.  The  fine 
mosslike  structure  is  a 
device  for  producing  as 
large  a  surface  as  pos- 
sible of  exposure  of  the 
blood  to  the  aerated 
water. 


Fig.  277. — Transverse  section  throug;h  a 
crawfish,  showing  the  g^lls  :  gcli,  gill 
chamber ;  //,  heart ;  /,  liver ;  »',  intes- 
tine ;  vs.,  blood  sinus.  The  arrows 
show  the  course  of  the  blood. 


Breathing. — The  exchange  of  water  is  effected  partly 
involuntarily  by  ciliary  currents,  partly  in  some  by  the 
action  of  the  maxillipeds,  and  partly — i.  e.,  when  in  active 
locomotion — by  the  movements  of  the  limbs,  and  there- 
fore of  their  appendages,  the  gills. 
27 


400   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

In  all  animals  violent  motion  causes  increased  respi- 
ration, but  the  mechanism  may  be  different.     In  verte- 


FiG.  278. — Side  view  of  the  circulatory  and  respiratory  system  of  a  crawfish,: 
h,  heart ;  gg,  the  gills.     The  arrows  show  the  course  of  the  blood. 


brates  increased  motion  causes,  of  course,  increased 
waste  in  the  blood.  This  in  time  stimulates  the  respira- 
tory centers  (medulla),  and  determines  through  the 
pneumogastric  nerve  increased  action  of  the  respiratory 
muscles.  In  crustaceans  the  increased  motion  produces 
increased  respiration  not  only  through  the  mediation  of 
the  nervous  system,  but  also  directly  by  the  motion  of 
the  gills  themselves. 

Circulation. — In  the  dorsal  region  behind  the  middle, 
immediately  beneath  the  carapace  and  between  the  points 

of  the  gills,  is  seen 

*r^^^^u^.         — =^..^  the  heart  [H,  Fig. 

276),   a   large  or- 
gan whose  pulsa- 
tions   throw    the 
^_^__  blood    fore     and 

f,^  r>-  1      •     .u  1  f    ^ft    through    the 

riG.  279. — Diagram  showing  the  general  course  of  * 

the  circulation  in  crustaceans:    G,   gills;  H,      large  dorsal  arte- 
heart.  ■         /         \  1 

ries   («  a)    to  the 

tissues.     Another  artery  goes  from  the  heart  downward 

to  form  the  sternal  artery,  and  thence  to  the  limbs  and 

lower  part  of  the  body  (Fig.  277).     From  the  tissues  the 


BLOOD    SYSTEM. 


401 


blood  is  gathered  into  large  reservoirs  (blood  sinuses), 
one  on  each  side,  running  along  near  the  base  of  the 
limbs.  From  these  sinuses  the  blood  is  again  taken  up 
by  the  veins,  to  be  distributed  to  the  gills  (Fig.  278)  and 
oxidized,  and  thence  to  the  heart,  to  be  again  distributed 
to  the  tissues.  Fig.  279  is  a  schematic  diagram  showing 
the  general  course. 

It  is  plain  that  the  heart  in  this  case  is  physiologic- 
ally a  systemic  heart,  since  it  contains  only  oxidized  blood, 
and  throws  it  to  the  tissues. 

MOLLUSCA. 

Acephala,  or  Bivalves. — Take  as  an  example  the 
river  clam  {^Anodonta).  Fig.  280  is  a  longitudinal,  and 
Fig.  281  is  a  transverse  section.     The  gills,  two  on  each 


Fig.  280. — Vertical  longitudinal  section  of  Anodonta  :  ptn,  am,  p)osterior- 
anterior  adductor  muscles  ;  w,  mouth  ;  st,  stomach  ;  c<e,  cascum  ;  //, 
heart ;  /,  foot ;  G,  gills  ;  ma,  mantle. 


side,  are  cellulated  sacs,  very  much  divided,  to  produce 
large  surface  of  contact  with  the  aerated  water. 

Breathing. — Ciliary  currents  pass  from  behind  for- 
ward through  the  gills,  determining  oxidation  of  the 
blood,  and  thenceforward  to  the  mouth  for  alimenta- 
tion, as  already  explained  (page  338),  and  then  backward 


402 


PHYSIOLOGY    AND   MORPHOLOGY    OF   ANIMALS. 


to  carry  away  excretions.  In  siphonated  bivalves,  as 
already  seen,  the  currents  pass  down  one  siphon  and  out 
the  other. 

Circulation :  Heart. — In  the  longitudinal  section  (Fig. 
280)  H  is  the  heart,  through  which  runs  the  intestine,  i, 
on  its  way  to  the  vent.  The  heart  consists  of  one  ven- 
tricle, and  usually  of  two 
M  auricles.      The  relation  of 

the  heart  to  the  gills  is  seen 
in  Fig.  282.  The  blood 
from  the  ventricle  is  thrown 


Fig.  281. — Transverse  section  of 
Anodonta  :  v,  ventricle  ;  au^  auri- 
cle ;  vs,  venous  sinus  ;  ;«,  mantle  ; 
br,  branchiae  or  gills  ;  y,  foot ;  /^, 
kidneys.  The  arrows  show  the 
course  of  the  circulation. 


Fig.  282. — Diagram  of  heart  and 
branchiae  of  a  bivalve,  viewed 
from  above  :  v,  ventricle ;  au, 
auricle ;  br,  branchia. 


fore  and  aft  to  the  tissues,  thence  gathered  from  capil- 
laries by  veinlets  and  emptied  into  several  sinuses  scat- 
tered in  different  parts  of  the  body.  From  these  it  is 
taken  up  by  the  branchial  vessels,  which  distribute  it  to 
the  gills,  where  it  is  aerated,  and  thence  by  branchial 
veins  to  the  auricles  on  each  side  and  to  the  ventricle, 
which  again  throws  it  in  the  tissues.  A  schematic  dia- 
gram which  represents  this  would  be  similar  to  that  used 
for  crustaceans  (Fig.  279),  except  for  the  addition  here 
of  the  auricle,  but  the  details  of  the  circulation  are,  of 


BLOOD   SYSTEM. 


403 


course,  quite  different.  Like  the  crustacean,  also,  the 
heart  is  a  systemic  heart — i.  e.,  aerated  blood  fills  the 
heart  and  is  distributed  to  the  tissues. 

Gastropods,  Univalves. — Take,  for  example,  a 
snail.  These  are  air  breathers.  As  already  said  (page 
340),  there  are  four  openings  of  the  body  in  front. 
These  are  (i)  the  mouth,  (2)  the  genital  opening  in  the 
immediate  vicinity,  (3)  the  vent,  and  (4)  the  pulmonic 
opening  beneath  the  shell  on  the  right  side.  We  are 
concerned  now  only  with  this  last.  It  opens  into  a  sac 
immediately  beneath  the  anterior  upper  portion  of  the 
shell  (pulmonic  sac),  on  the  interior  of  which  are  pro- 
fusely distributed  capillary  blood  vessels. 

Breathing. — The  change  of  air  seems  to  be  effected 
by  a  muscular  arched  membrane  just  beneath  the  lung 
sac,  which  may  be  compared  to  a  diaphragm.  The  con- 
traction of  this  membrane  lowers  the  arch,  expands  the 
lung  sac,  and  draws  in  the  air.  This  contraction,  of 
course,  compresses  the  viscera, 
on  which  it  rests.  On  the  relaxa- 
tion of  the  membrane  the  natural 
elasticity  of  the  compressed  vis- 
cera lifts  the  arch  and  expels 
the  air. 

Circulation.  —  The  heart  has 
three  chambers,  two  auricles  and 
a  ventricle.  Contraction  of  the 
ventricle  throws  the  blood  to  the 
tissues,  whence  it  is  gathered  into 

the  sinuses,  and  thence  it  is  again  taken  up  and  dis- 
tributed over  the  inner  surface  of  the  lung  sac,  where 
it  is  aerated  and  then  returned  to  the  heart,  to  be  dis- 
tributed again  to  the  tissues  as  aerated  blood. 

We  have  taken  the  higher  air-breathing  forms  of 
snails  and  slugs,  but   in  water  breathers,  of  course,  we 


Fig.  283. — Diagram  of  heart 
and  branchiae  of  a  Gastro- 
pod :  v\  ventricle  ;  au,  au- 
ricle ;  br,  branchiae. 


404 


PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


have  gills  instead  of  lung  sacs  and  water  breathing  in- 
stead of  air  breathing.  In  Fig.  283  we  show  in  diagram 
the  relation  of  heart  and  gills  in  these. 

In  many  lower  forms  [Nudibra/ichiafa)  the  gills  con- 
sist of  external  tufted  projections  from  the  skin,  waving 
in  the  water. 

Cephalopoda. — These  are  the  highest  of  mollusca. 
They  are  always  water  breathers,  but  their  gills  are 
quite  complex  and  efficient.  The  gills  of  these  animals 
lie  on  each  side,  in  the  dorsal  region,  and  the  breathing 
is  by  the  contraction  of  the  hol- 
low water-filled  muscular  man- 
tle, which  throws  out  the  water 
through  the  siphon  (see  Fig.  226, 
page  341),  and  its  own  elasticity 
restores  again  its  form  and  draws 
Fig.  284.^Diagram  of  heart     j,-,  ^^e  fresh  water. 

and  gills  of  dibranch  Ceph- 

alopod :  z;,  ventricle  ;  au.  Circulation.  —  In     dibranchs 

auricle ;  br.  branchia;.  ,  ■ , ,     ,  ^  11  11 

(two-gilled)    or    naked    cephalo- 

pods,  such  as  the  squid  and  cuttlefish,  there  are  two  auri- 
cles, one  to  each  gill;  in  tetrabranchs  (four-gilled)  or 
shelled  cephalopods,  such  as  the  nautilus,  there  are  four 
auricles.  In  all  there  is  but  one  ventricle.  The  relation 
of  the  heart  to  the  gills  is  seen  in  Fig.  284. 

ECHINODERMS. 

As  an  example  of  these  we  take  the  starfish  {As- 
terias) . 

Respiration. — The  respiration  of  these  is  performed 
in  two  ways:  i.  The  whole  body  is  hollow  and  the  body 
cavity  is  only  partially  filled  with  the  viscera,  leaving  a 
large  perivisceral  space  filled  with  water.  Contraction 
of  the  whole  body  presses  out  the  water  through  a  mul- 
titude of  pores  (Fig.  285,  a  «),  while  fresh  water  is  drawn 
HI  by  the  restoration  of  the  body  form.     The  conta-ned 


BLOOD   SYSTEM. 


405 


water  is  stirred  about  by  ciliary  currents  and  the  blood 
in  all  the  viscera  aerated.     2.  The  second  method  is  by 


TTi  ^^^p^y..^y 


Fig.  2S5. — Section  through  one  arm  of  a  starfish  :  si,  stomach  ;  >«,  mouth  ; 
cp,  caecal  pouches  ;  a/iip,  ampullae  ;  i/,  tube  feet ;  cr,  circular  vessel  or 
heart  from  which  go  the  blood  vessels ;  a  a,  openings  into  the  perivis- 
ceral cavity. 


the  vesicles  of  the  ambulacra.  On  the  underside  of  each 
of  the  five  arms  there  is  a  longitudinal  space  perforated 
with  rows  of  holes  (ambulacra),  through  which  project 
hollow  tentacles,  which  are  used  for  walking  (tube  feet). 
Each  hollow  tentacle  is  connected  with  a  vesicle  within 
(ambulacral  vesicle,  or  ampul- 
la), so  that  corresponding  with 
the  rows  of  ambulacral  tenta- 
cles on  the  outside  there  are 
rows  of  ambulacral  vesicles 
within  (Fig.  285  and  Fig.  286). 
These  vesicles  are  connected 
with  the  aquiferous  system, 
characteristic  of  echinoderms, 
in  such  wise  that  they  can  be 
filled  with  water  and  again  par- 
tially emptied  and  the  water 
changed. 

Circulation  and  Aeration. — The  heart  in  these  is  a  pul- 
sating organ,  extending  from  a  vascular  ring  under  the 
dorsal  skin  above  to  another  vascular  ring  surrounding 


Fig.  286. — Transverse  section  of 
an  arm  of  a  starfish  :  amp, 
ampulla  ;  tj\  tube  feet.  (Aft- 
er Parker.) 


4o6   PHYSIOLUGY   AND    MORPHOLOGY    OF   ANIMALS, 

the  mouth  below  (cv,  Fig.  285).  From  these  rings  go 
vessels  to  the  end  of  each  arm,  which  distribute  blood 
to  all  the  viscera.  This  blood  is  aerated  on  the  spot  by 
the  water  in  the  perivisceral  cavity.  Moreover,  a  por- 
tion of  the  blood  is  specially  aerated  by  distribution  on 
the  ambulacral  vesicles. 

In  addition  to  these  two  methods  the  highest  echino- 
derms,  such  as  the  Echinus,  have  tufted  external  gills 
attached  around  the  mouth. 

CCELENTERATA. 

Thus  far  we  have  a  distinct  blood  system.  The  three 
systems — food  system,  blood  system,  and  respiratory 
system — are  well  differentiated.  But  in  the  coelenterates 
these  three  are  not  yet  completely  separated.  The  di- 
gested food  serves  as  blood,  and  the  digestive  system  as 
blood  system.  Neither  is  there  any  respiratory  system 
distinct  from  either,  for  the  aeration  of  the  tissues  takes 
place  through  the  contact  of  water  on  the  outside  and 
ciliary  circulation  of  the  food  mixed  with  water  on  the 
interior  surface. 

Taking  the  polyp  (actinia)  as  example,  as  already 
shown  (Fig.  232,  page  344),  food  taken  into  the  stomach, 
s,  is  retained  by  pyloric  contraction  until  digested,  then 
dropped  into  the  general  cavity,  and  mixed  with  fresh 
sea  water.  The  mixture  is  stirred  about  by  ciliary  cur- 
rents and  directly  aerates  the  tissues.  Also  the  tissues 
are  similarly  aerated  by  contact  with  sea  water  on  the 
whole  exterior  surface. 

PROTOZOA. 

Finally,  in  the  protozoa  there  is  no  circulation  of  any 
kind,  nor  is  there  any  interior  aeration,  but  the  living 
protoplasm  is  directly  aerated  only  by  contact  of  water 
with  the  external  surface. 


BLOOD   SYSTEM. 


407 


This  kind  of  aeration  is  sufficient  for  animals  so  low 
in  the  scale  of  life,  and  especially  of  so  small  size,  for  in 
these  small  animals  the  surface  is  large  in  proportion 
to  the  bulk.  But  with  increasing  size,  since  bulk  in- 
creases as  the  cube  while  surface  only  as  the  square  of 
diameter,  it  is  evident  that  the  same  degree  of  aeration 
can  not  be  effected  without  some  device  to  increase 
the  surface  of  contact — i.  e.,  a  respiratory  organ  ;  and 
this  must  be  still  further  increased  when  the  organiza- 
tion is  higher  as  well  as  the  bulk  greater.  And  then, 
last  of  all,  for  greater  efficiency  the  whole  is  relegated 
to  an  ititernal  surface. 

The  same  is  true  of  the  nutritive  system  proper — i.  e., 
food  system  and  blood  system.  In  a  small  body  suf- 
ficient food  may  be  taken  directly  by  a  simple  surface, 
external  or  internal ;  but  in  a  large  body  the  absorbing 
surface  is  not  great  enough,  and  many  parts  are  too  far 
away  from  the  absorbing  surface.  There  must  be  a  sys- 
tem of  vessels  to  carry  nutriment  to  distant  parts.  This 
is  the  blood  system.  To  illustrate  :  In  a  small  island  no 
elaborate  system  of  internal  carrying  trade  is  necessary, 
for  all  parts  are  near  the  coast  where  products  are 
delivered.  But  as  the  island  becomes  larger,  and  espe- 
cially as  the  commercial  life  becomes  higher,  the  inter- 
nal carrying  trade  becomes  more  elaborate. 

INSECTS. 

It  will  be  remembered  that  we  passed  over  these  be- 
cause the  whole  plan  of  their  circulation  and  respiration 
is  entirely  different  and  wholly  out  of  the  line  of  gradual 
simplification  which  we  otherwise  find.  In  all  other 
classes  the  respiration  controls  the  course  of  the  blood, 
and  was  taken  up  first ;  but  in  insects  the  blood  system 
controls  the  character  of  the  air  system,  and  therefore 
the  blood  system  must  be  taken  up  first. 


4o8 


PHYSIOLOGY    AND   MORPHOLOGY   OF   ANIMALS. 


Blood  System. — Insects  are  very  highly  organized 
animals,  and  yet  their  blood  system  is  very  simple  and 
incomplete,  far  more  so  than  in  mollusca  or  even  echino- 
derms. 

Along  the  dorsal  aspect  of  the  body,  immediately  be- 
neath the  chitinous  shell,  there  is  a  long,  valvulated, 
pulsating  vessel — dorsal  vessel  (Fig.  287).  This  may  be 
called  the  heart.  The  valvules  are  so  arranged  as  to 
direct  the  course  of  the  blood  continualiy/(9;-7("«;-^.  This 
dorsal  vessel  divides  into  several  arterial  branches  at  its 
anterior  end,  and  receives  several  venous  branches  at 
its  posterior  end.  The  anterior  branches  discharge  into 
the  tissues  forward,  while  the  posterior  branches  suck 
in  from  the  tissues  behind.  This  is  apparently  all  that 
there  is  of  true  vessels.  In  verte- 
brates the  blood  system  is  a  closed 
system  of  pipes.  In  the  higher  in- 
vertebrates there  are  indeed  a 
number  of  reservoirs  scattered 
about  the  body  ;  but  still  the  blood 

^...,        .    ,      ,.„,,.       system  is  essentially  a  vascular  sys- 
1^  ■',','•  ^-'-■>-f  \<--     's'T         -^ 

]'<:<        {]      A'l'i       tem.     But  in  insects  there  are  no 
▼  '■',"  ~'->-'/iv"*'''  ',<■'■ 

vessels  at  all  except  the  large  ves- 
sels of  the  heart,  but  the  tissues 
are  everywhere  full  of  minute  in- 
tratissue  spaces  {laciincc)  connected 
with  one  another  in  all  directions. 
Now,  by  the  continuous  discharge 
of  blood  in  front  and  the  sucking 
up  of  blood  behind,  it  is  evident 
that  the  blood  must  work  round 
among  the  tissues  without  definite  channels,  but  in  a 
general  way  backward,  to  be  taken  in  again  into  the 
heart  and  forced  forward.  This,  therefore,  is  called  lacu- 
nary  circulation.    There  are  also  probably  valvular  open- 


Fig.  287. — Uiagram  show- 
ing the  heart  and  general 
course  of  the  circulation 
in  insects. 


BLOOD   SVSTEM. 


409 


^ 


\ 


ings  on  the  sides  of  the  heart  by  which  blood  may  be 
taken  in.  The  diagram  (Fig.  287)  is  an  attempt  to  rep- 
resent schematically  the  general  course  of  the  circula- 
tion. The  dotted  lines  represent  the  lacunary  circula- 
tion without  definite  vessels. 

The  difference  between  this  lacunary  circulation  and 
a  true  vascular  circulation  may  be  illustrated  by  an  irri- 
gation system.  The  circula- 
tion of  vertebrates  may  be 
compared  to  a  pump  and  a  sys- 
tem of  pipes  closed  through- 
out and  impermeable  until  the 
soil  to  be  irrigated  is  reached 
and  there  permeable.  After 
use  the  overplus  of  liquid  is 
again  gathered  into  imperme- 
able pipes  and  returned  to  the 
pump  to  be  again  used.  The 
circulatory  system  of  an  in- 
sect, on  the  contrary,  is  like 
a  pump  with  short  pipes  dis- 
charging on  the  soil  in  front 
and  sucking  up  from  the  soil 
behind.  But  the  soil,  instead 
of  being  penetrated  in  all  di- 
rections by  pipes  which  con- 
fine and  guide  the  currents,  is 

covered  with  little  pools  connected  by  channels.  Under 
these  conditions  the  water  would  work  around  in  an  in- 
definite way,  and  be  sucked  up  behind,  to  be  used  again. 

Respiratory  System. — Now,  it  is  this  peculiar 
mode  of  circulation  that  compels  the  very  exceptional 
kind  of  respiratory  apparatus.  Insects  are  active,  and 
somewhat  warm-blooded  animals,  and  therefore  require 
a  perfect  aeration  of  the  blood  ;  but  with  a  lacunary  cir- 


FlG.  288. — Tracheal  system  of  an 
insect :  sp  sp,  spiracles  ;  s  s, 
air  sacs. 


4IO  PHYSIOLOGY    AND    MORPHOLOGY    OF    ANLMALS. 


culation  this  is  impossible  in  any  localized  or ga.n.  In  a 
pipe  system  the  whole  of  the  blood  may  be  made  to  go 
through  an  organ  localized  in  some  part,  but  in  a  lacu- 
nary  system  a  local  organ  can  receive  only  its  small 
share  of  the  blood.  Evidently,  then,  if  the  blood  can 
not  go  to  the  air,  there  is  nothing  left  but  that  the  air 
must  go  to  the  blood.  This  is  exactly  what  it  does.  The 
air,  by  means  of  ramifying  tubes,  is  carried  to  every 
part  of  the  body  and  aerates  the  blood  on  the  spot 
everywhere. 

Air  Tubes;  Trachece. — On  the  margins  of  the  dorsal 
part  of  the  chitinous  shell  are  openings,  one  on  each  side 
of  each  movable  joint.  These  openings  (spiracles)  con- 
nect by  a  short 
tube  with  a  long 
lateral  tube  extend- 
ing on  each  side 
the  whole  length 
of  the  body  (Fig. 
288).  The  lat- 
eral tubes  connect 
across  the  body 
with  one  another 
by  several  trans- 
verse tubes;  and 
from  the  lateral  tubes  and  their  transverse  connections 
go  branches  and  sub-branches  until  the  minute  capil- 
lary branches  touch  every  cell  of  every  tissue.  All  these 
tubes  are  kept  open  by  a  spiral  thread  in  their  interior 
(Fig.  289). 

Now,  as  in  vertebrates,  in  proportion  to  the  vitality 
of  a  part  is  the  minuteness  of  the  distribution  of  the 
capillary  blood  vessels,  so  in  insects,  in  proportion  to 
the  vitality  of  any  part  is  the  minuteness  of  the  distri- 
bution of  the  air  tubes.     We  have  given  the  most  com- 


FlG.  2S 


-Tracheas  enlarged  to  show  the  spiral 
structure. 


BLOOD   SYSTEM. 


411 


mon   arrangement,  but   there   is  some  variation    in  this 
regard. 

Breathing. — If  vve  watch  a  hornet  or  wasp  at  rest, 
we  will  observe  a  back-and-forth  movement,  an  alter- 
nate lengthening  and  shortening,  of  the  abdomen.  This 
enlargement  and  contraction  of  the  body  cavity  draws 
in  and  expels  air.  We  at  once,  therefore,  see  why  oil  is 
so  fatal  to  insects.  It  covers  the  spiracles  with  a  film  and 
thus  suffocates  the  insect. 

SECTION    V. 

Lympliatic  or  Absorbent  System. 

Besides  the  blood  system,  there  is  another  system  of 
vessels  penetrating  the  tissues  everywhere,  which  may 
be  regarded  as  supplementary  to  the  blood  system.  It 
carries  not  blood,  but  a  clear  liquid  called  lymph,  and  is 
therefore  called  the  lymphatic  system.  It  is  not  a  circu- 
latory system,  but  purely  an  absorbent  or  drainage  sys- 
tem. We  have  already  seen  those  of  the  intestines — 
viz.,  the  lacteals  (page  326) ;  but  they  are  not  confined 
to  the  intestines,  but  occur  everywhere.  Nor  is  their 
function  even  in  the  intestines  confined  to  the  absorp- 
tion of  food  ;  they  absorb  many  other  things.  They  are 
far  less  understood,  both  as  to  their  distribution  and  as 
to  function,  than  the  blood  system,  for  they  are  diffi- 
cult to  see,  as  they  carry  colorless  liquid;  and  they  are 
difficult  to  inject  on  account  of  their  valvular  structure. 
We  treat  them,  therefore,  very  briefly. 

General  Description. — They  begin  by  blind  ex- 
tremities in  all  the  tissues,  but  especially  in  the  ab- 
dominal viscera,  forming  a  capillary  network  (Fig.  290), 
and  therefore  increasing  but  little  in  size  until  they 
reach  the  larger  trunks.  The  great  emptying  trunks  are 
(i)  the  thoracic  duct  on  the  left  side  of  the  backbone. 


412    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


This  we  have  already  seen  as  the  trunk  of  the  lacteals, 
but  it  is  also  the  trunk  of  much  else.  This  is  the  largest. 
(2)  Another  smaller 
duct  similarly  situ- 
ated on  the  }ight 
side.  These  two 
empty  into  the  cir- 
culation similarly — 


Fig.  290. — Lymphatics 
of  the  arm  :  agl^  axil- 
lary glands. 


Fig.  291. — Diagram  of  the  lymphatic  system  :  rch^ 
receptacle  of  chyle  ;  tlid,  thoracic  duct ;  th'd\ 
right  thoracic  duct.  The  arrows  show  the 
course  of  the  blood  in  the  jugular  and  sub- 
clavian veins. 


the  one,  as  already  explained,  into  the  angle  of  junction 
of  the  left  subclavian  and  left  jugular  veins,  the  other  at 


BLOOD    SYSTEM. 


413 


the  junction  of  the  right  subclavian  and  right  jugular 
veins.  The  former  (left  thoracic  duct)  drains  the  whole 
of  the  viscera,  the  whole  of  the  lower  limbs  and  the  left 
side  of  the  trunk,  the  left  arm,  and  the  left  side  of  the 
head,  while  the  latter  (right  thoracic)  drains  only  the  right 
side  of  the  trunk,  the  right  arm,  and  right  side  of  the 
head.     Fig.  291  is  a  diagram  showing  this. 

Structure. — The  valvular  structure  is  everywhere 
conspicuous,  and  gives  a  beaded  appearance  to  these 
vessels.  The  valves  open  toward  the 
main  trunks,  and  therefore  toward  the 
heart,  and  tend  to  prevent  backward 
movement.  This  is  the  more  neces- 
sary as  there  is  no  impelling  heart,  the 
force  of  motion  being  only  a  suction 
force  at  the  extremities.  Doubtless 
movements  of  the  body  and  pressure 
of  the  muscles  tend  to  force  on  the  con- 
tained fluid  (Fig.  292). 

Function. — Their  function  seems 
to  be :  I.  The.  absorption  of  waste.  2.  The 
absorption  of  excess  of  plasma  exuded 
from  the  capillaries  into  the  tissues. 
These  two  are  going  on  everywhere  and 
at  all  times.  3.  In  the  case  of  those  of 
the  intestines  (lacteals),  in  addition  to 
these  two  and  at  particular  times,  also 
the  absorption  of  the  digested  food.  In 
all  these  functions  they  are  assisted 
by  the  capillary  blood  vessels. 

Lymphatic  Glands.  —  In  the 
course  of  the  lymphatic  vessels  everywhere,  but  espe- 
cially the  bend  of  the  joints,  as  in  the  bend  of  the  elbow 
and  knee,  in  armpit  and  groin,  and  in  the  spaces  be- 
tween the  swelling  muscles,  as  on  the  side  of  the  neck, 


Fig.  292. — Enlarged 
section  of  a  lym- 
phatic vessel,  show- 
ing valves,  V. 


414 


PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


and  still  more  especially  in  the  visceral  cavities,  are 
found  glands.  We  have  already  spoken  of  those  of  the 
mesentery  in  connection  with  the  lacteals. 

Function  of  the  Glands. — The  functions  of  these 
glands  are  (i)  to  furnish  leucocytes  to  the  blood.  We  have 
already  spoken  of  this  function  in  connection  with  the 
mesenteric  glands,  but  now  add  that  all  the  lymphatic 
glands  have  this  important  function.  (2)  They  also,  as 
already  seen  in  the  case  of  the  mesenteric  glands,  seem 
to  confer  upon  the  lymph,  and  upon  the  blood,  perhaps 
through  the  presence  of  the  leucocytes,  the  property  of 
coagulation.  (3)  They  seem  also  to  have  the  power  to 
arrest  poisons  on  their  way  to  the  blood.  The  lymphatic 
vessels  seem  to  absorb  everything.  They  absorb  also 
poisons.  Such  absorbed  poisons  are  arrested  in  the 
glands,  producing  inflammation  and  suppuration  espe- 
cially m  the  armpit  and  groin.  The  poison  is  thus  elimi- 
nated, and  the  patient  saved.  They  suffer  vicariously 
to  save  the  body. 

Comparative  Morphology  of  the  Lymphatic 
System. — This  system  is  found  only  in  vertebrates.  The 
distinctive  functions  of  the  two  systems — blood  system 
and  lymph  system — have  not  yet  been  differentiated  in 
invertebrates.  The  blood  system  performs  the  func- 
tions of  both. 

In  lower  vertebrates  the  lymphatic  vessels  become 
much  more  distinct  than  in  man;  and  in  amphibians — 
e.  g.,  in  frogs — we  find  even  propelling  organs,  lymphatic 
hearts.  Two  of  these  are  found  in  the  sacral  region  and 
two  it!  the  scapular  region.  They  are  also  found  in 
some  birds,  especially  in  the  embryo  of  birds.  These 
vessels  empty  into  the  blood  system  in  various  places. 

The  \ym\i\\2iUc  glands  seem  to  appear  first  in  birds,  or 
perhaps  in  Crocodilia. 

Mammals  in  this  regard  are  in  all  respects  like  man. 


CHAPTER    IV. 

KATABOLISM. 
SECTION    I. 
Introductory. 

Thus  far  we  have  treated  oi  food  preparation  and  dis- 
tributioti.  Now  we  take  up  tissue  decomposition  and  waste 
elimination.  Thus  far  the  processes  are  ascensive  and 
distributive;  those  now  to  be  discussed  are  descensive 
and  eliminative.  In  a  word,  thus  far  we  have  had  to  do 
with  anabolism.     Now  we  take  up  katabolism. 

Introductory. — Exchange  of  matter  with  the  exter- 
nal world  can  take  place  only  through  an  external  sur- 
face or  an  infolding  of  an  external  surface.  Foreign 
commerce  can  take  place  only  through  a  coast  line  or  an 
infolded  coast  line  or  bay.  Many  of  the  so-called  inte- 
rior surfaces,  such  as  the  stomach,  the  intestines,  the 
lungs,  the  bladder,  etc.,  are  examples  of  such  infoldings 
of  the  exterior  surfaces.  Real  interior  surfaces — i.e., 
surfaces  which  have  no  connection  with  the  external 
world — are  found  in  the  cavities  of  the  blood  system  and 
of  the  nervous  system  (brain),  and  also  in  the  pleural 
and  peritoneal  cavities.  We  repeat,  then,  that  exchange 
with  the  external  world  can  take  place  only  through  an 
external  surface  or  an  infolding  of  the  same. 

\x\  plants  it  takes  place  on  a  directly  external  surface 
— on  an  exposed  coast  line.  Air  containing  food  bathes 
the  surface  of  the  leaves,  and  water  containing  food  the 
2S  415 


4i6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

surface  of  the  roots.  But  in  all  atiimals  a  part  at  least 
of  the  exchange  is  through  an  infolded  surface,  or  a  bay 
or  harbor.  In  the  lowest  animals  (^Protozoa),  absorp- 
tion or  imports  are  through  an  infolded  surface  or  bay, 
ih^  stomach  j  but  elimination  or  exports  are  still  through 
a  directly  external  surface,  an  exposed  coast  line.  In 
the  highest  animals  all  exchange,  both  imports  and  ex- 
ports, are  through  an  infolded  surface. 

In  going  up  the  scale  of  life  there  are  three  progres- 
sive changes  in  this  regard,  i.  The  relegation  of  more 
and  more  of  exchange  to  an  infolded  surface  or  bay 
until  a// is  thus  relegated.  2.  The  gradual  differentiation 
of  the  several  kinds  of  functions,  both  absorptive  and 
eliminative  (which  in  the  lowest  animals  are  performed 
in  every  part  alike),  and  their  localization,  each  in 
its  own  separate  place.  3.  The  increasing  complexity 
of  the  infolding  until  it  becomes  almost  inconceivably 
great.  If  the  simple  infolding  may  be  compared  to  a 
bay  or  harbor,  then  the  infolding  of  this  again  may  be 
compared  to  the  slips  and  docks  of  the  harbor. 

Now,  every  such  surface  through  which  exchange 
takes  place,  as  already  said  (page  20),  no  matter  how 
complex,  is  covered  with  a  pavement  of  living  nucleated 
epithelial  cells,  through  the  agency  of  which  the  ex- 
change takes  place.  Such  a  complexly  infolded  surface, 
covered  with  epithelial  cells  webbed  together  by  con- 
nective tissue  and  invested  and  isolated  by  fibrous  or 
serous  membrane,  constitutes  an  organ  absorptive  or 
eliminative.  The  extreme  complexity  is  especially  char- 
acteristic of  eliminative  organs,  and  among  these  the  most 
complex  of  all  is  the  lungs. 

Secretion  versus  Excretion. — Now,  elimination 
and  eliminative  organs  are  of  two  kinds — viz.,  secretions 
and  excretions,  secretory  organs  and  excretory  organs. 
In  the  former  the  products  do  noi  pre-exist  in  the  blood, 


K  ATA  150  L  ISM. 


41/ 


but  are  iiiaiiufactui cd  out  of  blood  and  used  in  the 
economy  of  the  animal  mainly  in  the  preparation  of 
food.  In  the  latter  the  products  pre-exist  in  the  blood, 
are  poisonous  to  the  blood,  and  must  be  removed.  In 
the  former  there  is  first  manufacture  and  then  elimina- 
tion of  a  useful  product.  In  the  latter  there  is  simple 
elimination  of  a  hurtful  product.  Salivary  glands,  pep- 
tic glands,  pancreas,  and  mammary  glands  are  examples 
of  secretory  organs.  The  lungs  and  the  kidneys  are 
the  best  examples  of  excretory  organs.  The  liver  is  pe- 
culiar and  of  a  mixed  character.  The  pure  secretory 
organs  connected  with  the  process  of  food  preparation 
we  have  already  discussed.  They  are  concerned  with 
anabolic  processes.  We  are  now  concerned  with  the 
purely  eliminative  or  excretory  organs,  for  these  belong 
to  katabolism.  The  liver,  being  mixed  in  its  functions, 
will  be  taken  up  later.  By  far  the  most  important  of  all 
the  katabolic  processes  is  that  of  respiration. 

SECTION    II. 

Function  of  Respiration. 

We  have  already  given  the  morphology  of  the  respira- 
tory organs,  because  this  could  not  be  separated  from  an 
account  of  the  circulation.  But  the  physiology  of  respi- 
ration— i  e.,  its  function  in  animal  economy,  its  relation 
to  katabolism — is  the  same  in  all  animals  and  must  be 
taken  now. 

We  have  already  seen  (page  358)  that  the  general 
purpose  of  respiration  is  the  aeration  of  the  blood,  or, 
more  specifically,  the  exchange  of  CO2  of  the  blood  for 
oxygen  of  the  air.  Thus  much  it  was  necessary  to  assume 
in  order  to  understand  the  course  of  the  circulation  and 
the  changes  in  the  blood  in  that  course.  We  must  now 
explain  the  essential  nature  of  the  function  and  its  neces- 


41 8    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 

sity.     For  greater  clearness  we  throw  the  explanation 
into  the  form  of  several  propositions. 

t.  The  Chemistry  of  Respiration. — The  essential 
chemical  nature  of  respiration  is  an  oxidation  of  food  and 
waste  in  the  blood.  Now,  oxidation  is  a  process  of  com- 
bustion. Respiration  is,  therefore,  a  burning  of  food 
and  waste  ;  more  specifically,  it  is  a  process  of  union  of  O 
of  the  air  with  C  and  H  of  the  blood,  and  the  formation 
of  CO2  and  HgO,  which  are  returned  to  the  air.  The 
fouling  of  the  air  by  respiration  is  due  mainly  to  the 
accumulation  of  the  eliminated  CO3. 

Proof. — The  proof  that  oxygen  is  consumed  and  CO2 
eliminated  in  respiration  is  so  familiar  that  it  is  unneces- 
sary to  give  it  except  in  bare  outline,  (a)  Thrust  a 
lighted  taper  into  a  gallon  jar,  and  it  burns  freely;  but 
blow  into  the  jar  through  a  tube  reaching  to  the  bottom 
until  the  jar  is  filled  with  the  expired  air  only,  and  again 
introduce  the  taper;  it  is  immediately  extinguished. 
{b)  Fill  a  test  tube  with  clear  limewater  and  blow  through 
the  water  with  a  glass  tube  for  some  time.  The  lime- 
water  becomes  milky  with  precipitated  lime,  carbonate 
{c)  Take  an  equal  bulk  of  pure  air  and  of  expired  air 
and  compare  their  composition  by  analysis.  It  will  be 
found  that  in  the  expired  air  a  certain  volume  of  oxygen 
has  disappeared  and  been  replaced  by  an  equal  or  nearly 
equal  volume  of  CO2.  So  much  for  the  COo.  Now  for 
the  HgO.  (^)  Breathe  on  cold  glass.  Immediately  the 
clear  glass  is  clouded  with  deposited  water. 

Now,  the  union  of  oxygen  with  carbon  and  hydrogen 
— i.  e.,  the  combustion  of  carbon  and  hydrogen,  always 
produces  heat.  This,  then,  is  the  source  of  animal  heat,  and 
the  difference  between  warm-blooded  and  cold-blooded 
animals  is  simply  a  difference  in  rate  of  combustion.  In 
warm-blooded  animals,  as  birds  and  mammals,  the  whole 
of  the  blood  is  oxidized,  the  surface  of  exposure  is  larger, 


KATABOLISM. 


419 


the  internal  fires  burn  fiercely,  and  therefore  the  tem- 
perature of  the  blood  is  high  and  independent  of  the 
temperature  of  the  exterior  medium.  In  cold-bloocJed 
animals,  such  as  reptiles,  amphibians,  and  fishes  among 
vertebrates,  and  in  nearly  all  invertebrates,  the  fires 
burn  so  low  that  the  temperature  of  the  blood  fol- 
lows somewhat  closely  that  of  the  exterior  medium. 
Every  stage  of  gradation,  however,  may  be  traced  be- 
tween them. 

2.  The  Purpose  of  the  Combustion. — The  pur- 
pose of  the  combustion  is  threefold:  {a)  To  remove 
waste.  The  waste  of  tissues  is  highly  poisonous  and 
must  be  quickly  removed  ;  most  of  it  is  removed  in  res- 
piration by  burning  it  up  and  changing  it  into  gaseous 
CO2  and  HgO  vapor,  which  are  readily  exhaled  from  the 
lungs.  {U)  A  still  more  fundamental  and  necessary  pur- 
pose of  combustion  is  \.\\&  generation  of  force.  This,  in- 
deed, is  the  origin  of  the  vital  force,  as  it  is  that  of  the 
force  of  the  steam  engine,  (r)  A  third  purpose  is  the 
generation  of  heat — i.  e.,  animal  heat.  This,  however,  is 
of  secondary  importance.  In  the  animal  machine,  as  in 
the  steam  engine,  the  real  purpose  is  force,  and  the  heat 
is  a  necessary  concomitant.  In  the  animal  body  the  heat 
is  sometimes  a  comfortable  (in  cold  weather),  sometimes 
an  indifferent,  and  sometimes  a  distressing  (in  hot 
weather)  concomitant.  But  we  can  not  get  the  force 
without  the  heat.  It  is  worthy  of  note,  however,  that 
from  this  point  of  view  the  animal  body  is  a  far  more 
efificient  machine  than  any  engine  ever  constructed — 
i.e.,  of  the  heat  of  combustion,  a  far  greater  proportion 
is  converted  into  force. 

3.  The  Fuel. — Again,  the  fuel  used  in  combustion 
is  of  three  general  kinds — viz.,  the  amyloids  and  fats,  the 
albuminoid  excess,  and  the  7vaste.  {a)  The  amyloids  and 
fats   consist   only  of   C,  H,  and   O,   and  therefore  these 


A20    PHYSIOLOGY   AND    MORPHOLOGY    OF    ANIMALS. 

burn  into  CO3  and  H2O  without  residue,  and  are  elimi- 
nated by  the  lungs  alone.  They  do  not  form  tissue. 
They  are  used  for  fuel  only,  {b)  Albuminoid  excess — 
i.  e.,  albuminoid  over  and  above  what  is  necessary  for 
tissue  building,  both  repair  and  growth,  {c)  Waste  tissue. 
These  last  two — i.  e.,  albuminoid  food  excess  and  waste 
— consist  of  C,  H,  O,  and  N,  and  often  a  small  quantity 
of  sulphur  and  phosphorus.  They  are  not  wholly  com- 
bustible into  CO2  and 
H2O,  and  wholly  elim- 
inable  by  the  lungs.  In 
CQtKO^  ^'^^^-XZ^iorea  burning  they  leave  an 
Fig.  293.-Diagram  showing  the  splitting      incombustible     residue 

of  albuminoids  in  a  combustible  and       tO  be  eliminated  by  the 
incombustible  portion.  ,„, 

kidneys.  1  hey  are,  as 
it  were,  j^///  into  two  parts,  a  combustible  and  an  incom- 
bustible. The  combustible,  consisting  of  the  larger  por- 
tion of  the  C,  H,  and  O,  is  eliminated  by  the  lungs,  but  a 
portion  of  the  C,  H,  and  O,  together  with  the  whole  of 
the  N,  is  eliminated  as  urea  by  the  kidneys.  This  is 
diagrammatically  represented  by  the  formula  (Fig.  293), 
in  which  the  line  a  b  represents  the  line  of  splitting. 
Therefore  these  two  organs,  the  lungs  and  the  kidneys, 
are  complementary  to  one  another.  They  divide  the 
albuminoid  food  excess  and  the  waste  between  them. 
But  the  amyloids  and  fats  are  disposed  of  only  by  the 
lungs. 

We  have  seen  that  the  three  kinds  of  fuel  used  are  (i) 
amyloids  and  fats;  (2)  albuminoid  excess;  and  (3)  waste. 
Now,  since  in  the  mature  body  the  repair  just  balances 
the  waste.,  it  is  evident  the  fuel  burned  is  exactly  equiva- 
lent to  the  whole  of  the  food. 

4.  But  the  question  occurs:  (i)  How  is  force  created 
by  combustion  ?  and  especially  (2)  How  can  force  enough 
be  generated  not  only  to  do  the  work  of  repair  and  main- 


KATABOLISM.  42 1 

tenance,  but  also  for  growth  and  activities  of  all  kinds  ? 
These  are  obviously  the  questions  most  fundamental  in 
physiology. 

In  answer  to  the  first  question  we  may  say  that  in 
the  passing  of  matter  from  a  more  complex  to  a  simpler 
condition,  from  a  more  unstable  to  a  stabler  condition, 
as  in  combustion  or  in  organic  decomposition,  force  or 
energy  is  liberated  which  is  converted  partly  into  heat 
and  partly  also  into  other  forms  of  energy,  mechanical 
or  vital.  To  illustrate  :  Matter  on  a  high  plane  (organic 
matter)  running  down  (katabolism)  to  a  lower  plane 
(COo  and  HoO)  generates  force  to  raise  (anabolism)  other 
matter  (food)  from  a  lower  to  a  higher  plane  (tissue). 
Thus  while  di  large  part  oi  the  force  oi  plant  life  (viz.,  the 
creation  of  organic  matter)  is  derived  from  the  sun  in 
the  form  of  light,  the  n'hole  of  the  force  of  animal  life  is 
generated  by  the  katabolic  process  going  on  in  the 
body. 

But  it  will  be  objected  that  a  certain  amount  of  mat- 
ter running  down  can  do  no  more  than  raise  the  same 
amount  of  matter  the  same  height.  The  whole  force 
of  u<aste  is  consumed  in  repair  and  nothing  is  left  over 
for  the  other  activities  of  the  body.  This  objection  is 
embodied  in  the  second  question.  The  answer  to  it 
is  (i)  that  in  case  of  waste  tissue,  the  running  down 
is  to  a  much  lower  plane  (COg  and  HoO)  than  that 
from  which  the  lifting  took  place  (food).  Therefore 
the  running  down  of  say  one  pound  of  tissue  to  CO3 
and  H3O  will  easily  lift  one  pound  of  albuminoid  food 
to  the  plane  of  living  tissue  and  leave  much  force  over 
for  activities  of  all  kinds.  (2)  There  is  also  much 
food — viz.,  all  the  amyloids  and  fats  and  all  the  albu- 
minoids in  excess  of  that  necessary  for  repair — that 
runs  down  and  generates  force  without  expending  its 
force  in  liftinjr  at  all. 


422    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

I  have  constructed  the  following  diagram  to   illus- 
trate this  process  (Fig.  294)  : 


Animal  Life 


„      ,      ( Albuminoids 
r.     Food 

lAviylafds  i&fats_ 


Fig.  294. — Diagram  illustrating  the  generation  of  animal  heat  and 
animal  force. 


We  have  represented  here  three  planes  raised  one 
above  the  other,  viz.,  (i)  the  plane  of  the  mineral  king- 
dom, (2)  the  plane  of  food,  and  (3)  the  plane  of  living 
tissue.  The  plane  of  food  is  subdivided  into  a  lower  and 
a  higher.  The  lower  form  of  food — viz.,  the  amyloids  and 
fats — is  at  once  burned  without  residue;  the  whole  of  it 
runs  down  to  the  mineral  kingdom  COo  and  HgO  *  and 
generates  a  corresponding  amount  of  force.  The  higher 
form  of  food,  albuminoids,  if  in  excess  of  the  necessities 
of  repair  and  growth,  also  runs  down,  but  in  its  down- 
ward course  is  split  (see  Fig.  293  on  page  420)  into  (a)  a 
combustible  portion  which  is  burned  to  COg  and  HgO, 
and  eliminated  by  the  lungs,  and  {/>)  an  incombustible 
portion  which  is  eliminated  on  a  little  higher  plane  as 
urea  by  the  kidneys.  Another  portion  of  albuminoid 
food  is  raised  to  the  plane  of  living  tissue  for  repair 
and  growth,  and  after  remaining  on  that  plane  and 
playing  its   part   there  for    a   time,    also   runs  down  as 


*  In  the  diagram  we  have  used  only  CO9,  because  this  really 
gives  the  amount  of  force,  the  H  and  the  O  being  already  in  the 
food  in  proportions  forming  HjO,  and  therefore  supplying  no 
heat  or  force. 


KATABOLISM. 


423 


waste  to  be  disposed  of  in  a  similar  way — i.  e.,  partly  by 
the  lungs  and  partly  by  the  kidneys.  The  whole  process 
may  be  likened  to  a  current  in  a  siphon :  the  shorter  arm 
is  anabolism,  the  longer  arm  and  the  one  which  de- 
ter tiii  ties  the  w/ioie  current  is  kafabolism. 

5.  We  have  said  that  respiration  is  a  process  of  com- 
bustion in  which  oxygen  of  the  air  unites  with  C  and  H 
of  the  blood  and  produces  COo  and  HoO.  Now  exactly 
the  same  takes  place  in  the  burning  of  oil.  The  material 
is  the  same  (C  and  H) ;  the  process  is  the  same  (union 
with  oxygen  of  the  air) ;  and  the  product,  chemical  and 
physical,  is  the  same — viz.,  COg  and  HoO  and  heat. 
Moreover,  it  is  certain  that,  estimating  the  whole  force 
produced  in  terms  of  heat,  the  amount  of  heat  is  the 
same  in  the  two  cases.  But  in  the  case  of  the  animal 
body  the  heat  is  spread  over  a  larger  space  and  over  a 
longer  time,  and  is  therefore  less  intense. 

6.  Another  important  question  is,  Where  does  the 
combustion  take  place  ?  The  old  view  was  that  it  takes 
place  in  the  lungs,  and  that  this  organ  is  the  furnace  of 
the  animal  machine,  that  circulation  brings  the  fuel  and 
respiration  brings  the  air,  and  that  the  fuel  is  burned 
at  once  then  and  there.  It  is  now  known  that  this  is 
not  the  fact.  The  blood  brings  the  products  of  com- 
bustion (CO2)  to  the  lungs  to  be  eliminated.  Respira- 
tion brings  a  fresh  supply  of  oxygen  to  be  taken  by  the 
blood  to  the  place  of  combustion.  This  place  of  com- 
bustion is  in  the  capillaries  in  contact  with  the  tissues. 
Yet  neither  are  the  decomposing  tissues  burned  at  once, 
but  circulate  in  the  blood  and  undergo  changes  there 
before  they  are  finally  consumed  into  COo.  These 
changes  are  very  obscure  and  little  understood.  Some 
of  them  are  spoken  of  later. 

Thus  the  blood  is  a  reservoir  for  many  things.  It  is 
a  reservoir  for  oxygen,  which  circulates  in  it  until  used. 


424 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


3  Animals 


It  is  a  reservoir  for  COg  until  eliminated  by  the  lungs. 
It  is  a  reservoir  (or  food,  which  is  drawn  upon  and  re- 
supplied  by  digestion  and  absorption.  It  is  a  reservoir 
for  wasie  tissue  until  it  is  finally  prepared  for  combus- 
tion. In  a  word,  it  is  a  reservoir  both  for  material  and 
for  force.  It  acts  like  a  fly  wheel  in  the  animal  ma- 
chine. 

7.  The  Relation  of  Plants  to  Animals  in  regard 
to  the  Creation  of  Animal  Force.— It  is  seen  from 
the  diagram  (Fig.  294)  that  the  anabolic  /i/f,  deter- 
mined by  the  kat- 

->  -~^ abolic  descent,  de- 

gins  more  than  half- 
7vay  up  the  scale  of 
existence.  Other- 
wise there  could 
be  generated  only 
force  enough  to 
make  the  lift  and 
maintain  the  tis- 
sues, and  none 
would  be  left  over 
for  activities  of  all 
kinds.  The  ques- 
tion then  occurs, 
How  does  matter 
attain  this  half- 
way plane  ?  The 
answer  is:  It  is 
raised  tothat  plane 
by  sunlight  acting  on  the  green  leaves  of  plants.  Ani- 
mals must  feed  on  organic  matter,  but  they  have  no 
power  to  make  it.  It  is  prepared  ready  for  them  by 
plants.  The  reason  is,  animals  are  wholly  dependent  on 
katabolic  processes  within  themselves  for  the  generation 


Fig.  295. — Diagram  to  show  the  forces  of  circula- 
tion of  organic  matter. 


KATABOLISM.  425 

of  vital  energy,  and  therefore  the  katabolic  descent  must 
go  far  below  the  plane  from  which  material  is  drawn  by 
anabolism.  Plants,  on  the  contrary,  lake  much  energy, 
viz.,  that  expended  in  making  organic  matter,  directly 
from  the  sun  ;  an  energy  generated  by  a  katabolic  pro- 
cess, true — viz.,  decomposition  of  CO2 — but  determined 
by  an  extertial  force,  sunlight.  So  that  in  the  eternal 
circulation  of  matter  from  the  plane  of  minerals  to  that 
of  animal  life  and  back  again  to  the  plane  of  minerals 
the  anabolic  ascent  begins  in  plants  and  completes  itself 
in  animals;  but  the  katabolic  descent  completes  itself  at 
once  in  animal  katabolism.  So  that  the  whole  circula- 
tion may  be  represented  by  an  endless  chain,  as  in  the 
adjoining  figure  (295).  This  circulation  must  be  driven 
in  part  by  a  force  external  to  the  chain.  That  force  is 
the  sun.  It  is  evident  that  animal  force  in  activity,  in 
excess  of  maintenance  of  tissues,  is  the  equivalent  of 
the  excess  of  katabolism  over  anabolism — of  the  excess 
{2,  i)  of  the  katabolic  branch  of  the  siphon.  But  this 
is  exactly  equal  to  the  lift  by  the  sun  (/,  2).  Therefore 
animal  force  or  activity  is  equivalent  to  sun  force  in 
making  organic  matter. 

SECTION    III. 
T/:c  fCt'dfieys :    The  Organ  and  its  Fit  net /on. 

The  Organ. — We  have  already  said  that  the  lungs 
and  kidneys  are  the  two  great  organs  for  elimination  of 
the  products  of  katabolism.  They  are  also  correlative, 
since  they  divide  between  them  the  final  disposal  of  the 
albuminoid  food  and  the  waste.  The  kidneys,  therefore, 
must  be  our  next  subject. 

Place  and  Form. — The  kidneys  are  in  the  abdomi- 
nal cavity,  in  the  hollow  on  each  side  of  the  lumbar  ver- 
tebree,  and  just  below  the  roots  of  the  diaphragm.     The 


426   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


vca 


shape  is  very  like  a  kidney  bean,  and  they  lie  with  their 
concave    sides  toward  each  other.     The    blood    supply 

is  large  in  propor- 
tion to  the  size  of 
the  organ,  because, 
as  in  the  case  of 
the  lungs,  the  sup- 
ply is  not  mainly 
for  nutrition  of  the 
organ,  but  for  puri- 
fication of  the  blood 
(Fig.  296).  The 
blood  brought  back 
from  the  kidneys 
by  the  renal  vein 
is  the  purest  blood 
in  the  body,  for  it 
is  brought  to  the 
kidneys  as  bright 
blood  purified  of 
CO3  in  the  lungs, 
but  -  still  contain- 
ing urea  ;  but  now 

Fig.   296. — Showing  form  and  position  of  the      .      .  .^,.. 

kidneys:   ao,  aorta;    vca,  vena  cava  ascen-      it    IS    purincd  Ot    ItS 
dens;  ra,  renal  artery;    rv^  renal  vein;   tt,      ,     _„  •      -.l      virln^vc 

ureters ;  <5A  bladder.  urea  in  ttie  kidneys 

and  returned  to  the 
vena  cava  ascendens.  There,  however,  it  is,  of  course, 
again  mixed  with  impure  blood  from  other  tissues. 

Excretory  Duct. — The  excretory  ducts  (the  ure- 
ters) are  peculiar.  By  a  trumpet-shaped  mouth  each 
ureter  grasps  the  deeply  concave  part  (the  pelvis)  of 
the  kidney  so  as  to  receive  the  excretions;  then  passes 
down  on  each  side  as  a  tube  about  the  size  of  a  crow 
quill  and  about  fifteen  inches  long  and  enters  the  blad- 
der on  each  side  below  by  a  valvular  opening,   which 


KATABOLISM. 


effectually  prevents  regurgitation.  The  secretion  of  the 
kidneys  drips,  little  by  little,  continually.  It  accumu- 
lates in  the  bladder,  and  is  thence  from  time  to  time 
voided  through  the  urethra. 

Pelvis  of  the  Kidney. — If  the  ureter  be  cut  away, 
we  expose  a  deep  concavity  called  the  pelvis.  Its  whole 
interior  is  covered  with 
mammillary  protuber- 
ances, like  the  ends  of 
the  fingers  put  together. 
These  are  the  ends  of 
the  cones.  In  the  living 
animal  (chloroformed) 
we  may  see  liquid  oozing 
from  innumerable  pores 
on  this  surface,  collect 
into  drops,  and  run 
down.  These  pores  are 
the  openings  of  the  uri- 
niferous  tubules. 

Section. — By  longi- 
tudinal section  through 
the  pelvis  (Fig.  297)  it 
is  at  once  seen  that  there 
are  two  parts  of  the  kid- 
ney, differing  in  color 
and  structure — an  inner 
portion,  of  lighter  color 

and  radiated  structure,  and  an  outer  portion,  darker  and 
nonradiated.  The  former  is  the  tnedi/I/ary  and  the  latter 
the  cortical  portion.  The  excretion  takes  place  mainly 
in  the  cortical  portion  ;  the  medullary  mainly  transmits 
it  to  the  pelvis. 

Minute   Structure.  — Examined  with  a  microscope, 
the   medullary  portion  is    seen    to    consist   of    straight 


Fig.  297. — Section  of  the  kidney  showing 
structure  :  P,  pelvis  ;  u,  ureter  ;  /«, 
medullary,  and  c,  cortical  portion  ;  a  a, 
arteries. 


428   PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


tubes,  branching  and  becoming  more  numerous,  but  the 
branches  lie"  parallel,  and  give  rise  to  a  radiating  ar- 
rangement. In  the  cortical  portion,  on  the  contrary, 
the  tubes  are  much  convoluted,  and  finally  terminate  each 
in  a  vesicle,  which  is  filled  with  a  tuft  of  capillary  blood 
vessels,  looping  round  and  connecting  with  an  arteriole 

o/i  the  one  hand  ax\d  with  a 
veinlet  on  the  other  (Fig. 
298).       Blood    vessels   are 


Fig.  298. — Uriniferous  tubules  :  m, 
Malpighian  corpuscles  ;  gl,  glom- 
erules. 


Fig.  299. — Uriniferous  tubule  :  ttt, 
with  its  Malpighian  corpuscle  and 
glomerule  ;  a,  arteriole  ;  i\  veinlet. 


also  abundantly  distributed  among  the  tubules.  These 
terminal  vesicles  are  the  Malpighian  corpuscles,  and  the 
contained  vascular  tufts  glomendes. 

Now,  as  in  all  eliminative  organs,  so  here  the  whole 
is  lined  throughout  with  epithelial  cells  (Fig.  299).  The 
epithelial  membrane  passes  from  the  surface  up  the  ure- 
thra into  and  lines  the  bladder;  from  the  bladder  it 
passes  up  the  ureters  and  covers  the  pelvis.  It  then 
passes  up  and  lines  the  tubules  and  their  terminal  vesi- 


KATABOLISM. 


429 


cles.  In  fact,  the  kidney  is  little  else  than  a  mass  of 
such  tubes,  lined  with  epithelium,  webbed  together  with 
connective  tissue,  and  invested  with  peritoneal  mem- 
brane, and  then  the  whole  liberally  supplied  with  nerves 
and  blood  vessels.  The  epithelial  surface  thus  produced 
is  enormously  great. 

Function. — The  function  of  the  kidneys  is  apparent- 
ly twofold — the  ont  physical^  the  other  vital.  The  one  is 
a  purely  physical  and  therefore  nonselective  filtration,  the 
other  is  a  selective  excretion  of  a  particular  product  of 
katabolism — viz.,  urea.  The  one  is  probably  through  the 
capillary  tufts  of  the  terminal  vesicles,  and  apparently 
not  by  the  agency  of  the  ordinary  epithelial  cells  (cells 
here  are  squamous  epithelium) ;  the  other,  as  in  the  case 
of  all  the  secretions  and  excretions,  is  by  the  agency  of 
the  living  nucleated  cells  lining  the  tubes.  These  are 
glandular  epithelium.  By  the  former  are  eliminated  from 
the  blood  superabundant  water  and  salts  of  all  kinds, 
normal  or  accidental.  By  the  latter  is  removed  a  char- 
acteristic product  of  katabolism — urea.  This  is  the  spe- 
cial characteristic  function  of  the  kidneys. 

Composition  of  Urine. — The  secretion  of  the  kid- 
neys is  a  solution  of  urea  (sometimes  partly  replaced  by 
uric  acid)  and  of  various  salts — viz.,  sulphates,  phos- 
phates, chlorides  and  carbonates  of  potash,  soda,  and 
lime.  The  most  important  parts  are  the  urea  (or  its 
equivalent,  uric  acid),  together  with  the  sulphur  and 
phosphorus  oi  Vat.  sulphates  and  phosphates.  The  chlo- 
rides and  carbonates  are  constantly  present  in  the  blood, 
and  their  excess  eliminated  by  the  kidneys.  Urea  and 
to  some  extent  also  the  sulphates  and  phosphates  are 
products  of  katabolism.  The  essential  function  is  the 
elimination  of  N  and  a  little  S  and  P.  Of  the  elements  of 
albuminoids — viz.,  C,  H,  O,  N,  and  S  and  P,  as  already 
explained — the  larger  part  of  the  C,  H,  and  O  are  re- 


430  nivsiOLOGY  and  morphology  of  animals. 

moved  through  the  lungs  as  COg  and  HgO  ;  but  the  whole 
of  the  N,  P,  and  S,  together  with  enough  C,  H,  and  O  to 
form  with  the  N  the  substance  urea,  are  removed  by 
the  kidneys.     The  composition  of  urea  is  CH4N2O. 


I  Urea 
c  w         f  A         r  Chlor.  1 

Urine  =  Solution  of  -.     and         I    „     , 

,,  ,^  I  Garb.  f   1  -rr 

{  Salts  =  -  .  of  -;  K 

Sulph. 


t  Phos. 


Na 

K 

Ca 


The  manner  in  which  albuminoids,  both  food  excess 
and  waste,   are   divided   between   lungs  and  kidneys  is 
shown   in   Fig.  300,   already   given,   but    repeated    here 
slightly  modified.     The  amyloids  and  fats  are  not  elimi- 
nated   by   the    kid- 
.^  neys,  but  only  the 

lungs. 

It     is     evident, 
lungs^     ^"-^     ~XZ^Z^::^^ n.Lincys     then,  that  the  quan- 

FlG.  300. — Diagram   showing   the  division  of     tity    of    urea     elimi- 
albuminoids  between  the  lungs  and  the  kid- 
neys, nated   is  in  propor- 
tion to  the  quantity 
of  albuminoid  food,  and  is  therefore  greater  in  carnivores 
than  in  herbivores,  and  also  in   proportion  to  the  waste, 
and  therefore  to  the  work  done  or  degree  of  activity. 
The  estimation   of  the   urea   excreted   is   therefore   the 
most  convenient   mode   of  measuring  tuaste,  and  is  con- 
stantly used  for  this  purpose. 

When  urea  decomposes  in  the  presence  of  water  it  is 
wholly  converted  into  ammonium  carbonate. 

CH^XoO  +  2H2O  =  (H4N)oC03 
urea        -(-     water  =      am.  carb. 

Comparison  of  Lungs  and  Kidneys. — The  com- 
parison between  these  two  complementary  organs  is  in- 
teresting in  many  respects. 


KATABOI.ISM. 


431 


1.  We  have  alluded  (page  419)  to  the  importance  of 
the  quick  removal  of  the  products  of  katabolism.  Stop 
respiration,  and  death  by  blood  poisoning  takes  place  in 
five  to  ten  minutes.  If  the  function  of  the  kidneys  stops, 
death  by  blood  poisoning  (uraemia)  occurs  in  twenty-four 
to  forty-eight  hours.  See,  then,  the  much  greater  im- 
portance of  the  lungs. 

2.  It  is  on  account  of  this  supreme  importance  that 
in  the  higher  animals  the  whole  of  the  blood  passes 
through  the  lungs,  that  there  is  complete  double  cir- 
culation, and  that  blood  completely  purified  of  COg  goes 
to  the  tissues.  On  the  contrary,  only  a  portion  of  the 
blood  passes  through  the  kidneys;  the  renal  circula- 
tion is  only  a  branch  of  the  systemic  circulation,  and 
therefore  only  mixed  blood,  so  far  as  urea  purification 
is  concerned,  goes  to  the  tissues.  But  it  will  be  remem- 
bered that  the  same  is  true  of  the  pulmonic  circulation 
in  reptiles  and  amphibians. 

3.  Of  foods,  amyloids  and  fats  are  wholly  removed 
by  the  lungs,  while  the  albuminoids  are  divided  between 
the  lungs  and  the  kidneys  in  the  manner  already  ex- 
plained. Of  the  elements  of  organic  matter,  the  larger 
part  of  the  C,  H,  and  O  is  removed  by  the  lungs,  but 
the  whole  of  the  N,  S,  and  P  by  the  kidneys. 

4.  The  lungs  not  only  eliminate  C'Oo  and  HgO,  but 
take  in  O  for  combustion.  The  function  of  the  kidneys, 
on  the  contrary,  is  purely  eliminative.  The  eliminated 
product  in  case  of  the  lungs  is  the  result  of  oxidation  ; 
in  the  case  of  the  kidneys,  of  decomposition. 

5.  The  circulation  of  the  elements  of  organic  matter 

back  to  the  atmosphere  is  through  both  of  these  organs. 

The  return  of  C  in  the  form  of  COo  is  through  the  lungs ; 

the  return  of   N   in  the  form  of  ammonia  is  through  the 

kidneys    as    urea,  which,  as   we    have    seen,   is  quickly 

changed  into  carbonate  of  ammonia.     Thus  all   the  ele- 
29 


432 


PHYSIOLOGY   AND    MORPHOLOGY   OF  ANIMALS. 


ments  of  living  organisms  taken  from  the  atmosphere 
and  embodied  for  a  brief  time  are  again  returned,  and 
the  same  matter  is  worked  over  and  over  again  by  an 
eternal  circulation.  The  circulation  of  C  and  O  through 
the  atmosphere,  plants,  and  animals  back  to  the  atmos- 
phere is  represented  in  diagram  (Fig.  301,  repeated  from 
page  5).  It  is  seen  that  C  is  taken  in  the  form  of  CO2 
from  the  atmosphere  by  plants,  is  decomposed,  the  C 
fixed  in  organic  matter,  and  oxygen  returned  to  the  air. 
The  C  is  taken  from  plants  by  animals  as  food,  is  then 
combined  with  O  from  air  taken  in  respiration,  and  re- 
turned to  the  air  in  its  original  form  as  COg,  and  so  on 
continually. 

The  circulation  of  all  the  elements  of  organic  matter 
between  the  organic  kingdom  and  the  atmosphere  is 
represented  by  the  diagram  (Fig.  302).  The  food  of 
plants  consists  of  CO3,  HgO,  and  NH3.     These  are  taken 


^tffoi^^e^^ 


'^limaM 


P\a-n\S 


Fig.  301.  -Diagram  illustrating  the 
circulation  of  carbon  and  oxy- 
sen. 


Fig.  302. — Diagram  showing  the 
circulation  of  the  elements  of  or- 
ganic matter. 


from  the  atmosphere  and  embodied  in  organic  form  of 
both  plants  and  animals.  In  their  death  these  are  again 
returned  to  the  atmosphere  as  COo,  HgO,  and  NH3,  to 
be  again  taken  as  before.     Thus  the  same  small  quan- 


KATABOLISM. 


433 


tity    of  CO2  and  NH3  are  embodied   and   disembodied 
many  times  in  the  history  of  the  organic  kingdom. 

COMPARATIVE    MORPHOLOGY     OF     THE    KIDNEYS. 

Vertebrates :  Mammals. — Little  need  be  said  on 
the  mammahan  kidney.  The  organ  has  a  similar  posi- 
tion in  all  mammals,  and  in  most  a  similar  shape  and 
structure  to  that  of  man. 
In  the  ox  the  surface  is 
mammillated  as  if  there 
were  a  commencing  sepa- 
ration of  the  cones.  This 
separation  is  realized  in 
some  living  water  mam- 
mals, such  as  the  otter 
and  the  porpoise  (Fig.  303). 
But  in  these  cases  the  dif- 
ference is  not  significant. 
Each  cone  consists  of  a 
medullary  and  a  cortical 
portion,  and  they  all  dis- 
charge into  one  pelvis  and 
ureter. 

Birds. — The  first  important  change  in  this,  as  in  so 
many  other  characters,  is  found  in  birds.  In  these  (i) 
the  kidneys  are  not  yet  clearly  differentiated  into  two 
parts,  a  cortical  and  a  medullary,  having  different  func- 
tions. (2)  In  these  there  is  no  bladder,  but  the  ureters 
empty  into  a  cloaca  or  enlargement  of  the  rectum  just 
within  the  vent. 

Reptiles  and  Amphibians  (i.  e.,  all  cold-blooded 
land  vertebrates)  have  a  cloaca  \  but  some — e.  g.,  snakes 
and  lizards — like  birds,  have  no  bladder,  the  ureters 
emptying  directly  into  the  cloaca;  while  others — e.  g., 
tortoises  and  frogs — have  a  bladder.     In  these  cases  the 


Fig.  303. — Section  of  the  kidney  of  a 
porpoise,  showing  its  structure  : 
tt,  ureter.     (  From  Owen. ) 


434 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


ureters  empty  into  the  bladder  and  the  tirethra  into  the 
cloaca. 

The  diversity  in  structure  in  the  different  orders  of 
fishes  is  so  great  that  their  description  would  carry  us 
beyond  the  scope  of  this  work. 

Invertebrates  :  Insects. — The  essential  character 
of  the  kidneys — i.  e.,  its  significance  in  katabolism — is  the 
excretion  of  urea.  This  is  therefore  the  true  test  of  an 
organ  corresponding  functionally  to  the  kidneys.  Now, 
there  is  an  extremely  delicate  chemical  test  of  urea — viz., 
the  splendid//^;;/!'/*?  produced  by  certain  reagents.  It  is 
wholly  by  this  test  that  we  are  able  to  determine  the  renal 
organ  in  invertebrates.  In  this  way  it  is  determined 
that  the  so-called  biliary  tubes  of  insects  (Fig.  219,  bd, 
page  336)  are  also  their  kidney — i.  e.,  they  excrete  urea. 

In  crustaceans  the  so-cdAX^d,  green  glands  (Fig.  221, 
SSi  V^Z^  337)>  which  are  large  glands  near  the  base  of  the 
antennae  and  very  liberally  supplied  with  blood,  are  the 
kidneys.  But  it  is  very  noteworthy  that  in  addition  to  a 
peculiar  form  of  uric  acid  (carcinuric  acid)  there  is 
secreted  also  leucomaines*  which  is  the  first  product  of 
albuminoid  waste  in  higher  animals,  and  which  in  them 
is  split  into  a  combustible  portion,  for  use  as  fuel,  and 
an  incombustible  portion  excreted  as  urea  (pages  420 
and  430).  In  crustaceans  some  of  these  substances  is 
excreted  unutilized,  and  thus  wasted.  This  is  evidence 
of  low  organization. 

MoUusca. — By  similar  tests  renal  organs  have  been 
detected  in  different  classes  of  mollusca — viz.,  the 
"  spongy  bodies  "  of  cephalopods,  the  organ  of  Bojanus 
of  acephala  situated  at  the  base  of  the  gills  (Fig.  281, 
page  402),  and  the  lamellar  gland  near  the  pulmonic 
sac,  or  the  branchiae  of  gastropods. 

*  Marchal,  Rev.  Sci.,  li,  178,  1S93. 


KATABOI-ISM. 


435 


Below  these  the  renal  organ  has  not  been  detected. 
Probably  this  function  has  not  yet  been  differentiated 
from  others. 

Observe,  then  :  (i)  Only  a  little  way  down  the  verte- 
brate scale  [birds)  the  distinction  of  cortex  and  medulla 
is  lost.  (2)  Only  a  little  farther  down  the  animal  scale 
{insects)  the  functions  of  liver  and  kidneys  are  merged, 
although  they  are  found  separate  even  lower  down. 
(3)  A  little  lower  down  [cn/s/acea)  the  function  of  the 
kidneys  is  incompletely  performed — i.  e.,  a  part  of  the 
waste  escapes  decomposition  and  is  thus  wasted  by  not 
being  utilized  as  fuel.  (4)  A  little  lower  down — i.  e., 
below  mollusks — this  organ  has  not  been  certainly 
found.     Probably  the  function  is  not  yet  differentiated. 

SECTION    IV. 
T/ic  Skin  and  its  Function. 

We  have  spoken  of  the  lungs  and  kidneys  as  the  two 
great  organs  which  share  the  elimination  of  katabolic 
products  between  them.  But  there  is  still  a  third,  the 
skin,  which,  though  less  important,  must  not  be  neg- 
lected. 

Function. — All  organs  and  functions  are  sympa- 
thetically related  to  one  another,  so  that  the  importance 
of  healthy  action  of  an  organ  is  not  to  be  measured  by 
that  of  its  more  obvious  and  distinctive  function.  The 
distinctive  function  of  the  skin  is  t/ie  moderation  of  the 
blood  heat  produced  by  continual  combustion,  by  the 
elimination  of  excess  of  water,  and  its  evaporation  on  an 
exposed  surface.  The  elimination  of  water  it  shares  with 
the  kidneys  and  is  complementary  with  it,  but  it  is 
peculiar  in  the  elimination  of  water  in  such  wise  as 
thereby  to  cool  the  blood. 

But  elimination  of  water  by  the  skin,  as  also  by  the 


4^6   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

kidneys,  is  of  two  kinds — physical  and  vital.  The  for- 
mer is  exhalation,  the  latter  excretion.  The  one  is  always 
insensible,  the  other  is  sensible — sweat. 

Exhalation. — Water  in  a  porous  earthenware  jar  or 
in  a  canvas  bag,  if  hung  up  in  the  air  in  a  hot,  dry 
climate,  keeps  cool,  and  even  grows  cooler.  The  water 
soaks  through  and  evaporates  on  the  surface,  but  never 
drips.  So  the  tissues  of  the  body  are  all  permeable  and 
filled  with  water.  Evaporation  from  the  surface  calls 
the  water  to  the  surface  as  fast  as  it  is  evaporated.  It 
can  not  drip  as  sweat,  because  it  is  called  there  only 
by  drying.  It  is  for  this  reason  that  in  very  arid  re- 
gions the  suffering  from  heat  is  not  very  great,  and 
sunstrokes  are  unknown,  although  the  thermometer  in 
the  shade  may  run  up  to  iio°  to  120°  F.  Of  course  the 
water  which  comes  out  and  evaporates  in  this  physical 
way  is  nearly  pure  water. 

Excretion. — But  besides  this  physical  exhalation, 
which  would  go  on  even  if  there  were  no  special  organs, 
there  is  also  the  excretion  of  srveat.  This  is  the  product 
of  the  sweat  glands.  It  is  not  called  out  by  evaporation, 
but  as  if  it  were  pushed  by  the  specific  activity  of  the 
glands,  and  may  exude  in  such  quantity  as  to  roll  down 
and  drip  from  the  skin.  This  also  spreads  and  evap- 
orates, and  thus  cools  the  surface.  This  is  not  pure 
water  like  the  other,  but  water  containing  many  salts 
with  a  little  oil  and  a  little  CO,  and  a  trace  of  urea. 

Structure  of  the  Skin.— The  skin,  as  already  ex- 
plained, is  composed  of  two  parts — viz.,  the  dermis  ox  trtie 
skin  and  the  epidermis.  The  dermis  is  a  mass  of  felted 
interlacing  fibers.  It  is  very  strong,  highly  organized, 
and  full  of  blood  vessels  and  nerves.  The  epidermis, 
which  alone  concerns  us  here,  is  continuous  with  the 
epithelium,  and,  as  to  its  truly  living  parts,  similarly 
constituted.      There  is,  however,   this   difference:  The 


KATABOLISM. 


437 


epithelium  consists  of  only  a  few  layers  of  living  cells, 
which  are  constantly  dying,  shedding,  dissolving,  and 
forming  the  slimy  mucus  which  covers  the  interior  sur- 
face and  again  renew- 
ing ;  while  in  the  epi- 
dermis the  lower  layer 
in  contact  with  the 
dermis  consists  also 
of  living  nucleated 
cells,  which  are  also 
constantly  dying  and 
renewing;  but  the  dy- 
ing cells,  instead  of 
dissolving  and  form- 
ing slime  and  leav- 
ing the  surface  still 
composed  of  living 
cells,    gradually    dry 

up,  mummify,  flatten  Fig.  304.— Section  through  the  skin,  show- 
mnrp    and    mnrp      anrl  '"^  its  structure:  ep,  epidermis,  and  w, 

more    ana    more,    ana  its  Malpighian  layer  ;  a-,  dermis  ;  j-^,  sweat 

finally      pass     off      as  gland  ;  jo',  sweat  duct ;  j/,  sense  papills  ; 

//,  hair  ;  bx\  blood  vessels. 

scales    of    the    scarf 

skin.  Thus,  besides  the  lower  layer  [Malpighian  layer) 
of  living  cells,  there  are  many  layers  in  various  stages 
of  dying  and  mummification.  This  mummified  part  is 
the  cuticle.  The  color  of  the  skin  is  in  the  lower  living 
layer.  A  blister  is  a  lifting  of  the  epiderm  from  the 
derm  and  an  accumulation  of  lymph  beneath. 

Sudorific  Glands. — Scattered  in  great  numbers  over 
the  surface  of  the  skin  are  found  pores  formed  by  the 
infolding  of  the  living  or  Malpighian  layer  of  the  epi- 
derm, forming  tubes  (sJ,  Fig.  304)  opening  on  the  sur- 
face. They  pass  through  the  dermis  into  the  subcutane- 
ous connective  tissue,  and  are  there  convoluted  into  a 
pellet,  sg.     These  are  the  sudorific  glands.     The  num- 


^^S    PiiVSlULOGY   AND    MORPHOLOGY    OF    ANIMALS. 

ber  of  the  pores  has  been  variously  estimated  at  from 
five  hundred  to  twenty-eight  hundred  to  the  square  inch, 
and  from  one  and  a  half  million  to  seven  millions  on 
the  whole  surface  of  the  body.  The  aggregate  length 
of  the  tubes  has  been  estimated  at  twenty-eight  miles.* 
Like  all  infolded  surfaces,  the  tubes  are  lined  throughout 
with  living  epithelial  cells,  which  preside  directly  over 
the  excretion.  Blood  vessels  furnishing  the  materials 
are  distributed  over  the  exterior  surface  of  the  tubes. 

It  is  quite  possible  that  the  number  and  aggregate 
length  of  the  uriniferous  tubes  may  be  as  great  as  that 
of  the  sudoriferous,  but  in  the  one  case  these  are  com- 
pacted into  an  organ  (the  kidney),  and  the  excretion 
gathered  into  a  reservoir  (the  bladder),  while  in  the 
other  they  are  spread  over  the  wide  surface  of  the  skin. 
The  reason  is  obvious.  The  purpose  of  cooling  the 
blood  by  evaporation  could  only  be  subserved  by  a  large 
surface  of  direct  exposure. 

Lungs,  Kidneys,  and  Skin  compared. — AH  three 
of  these  eliminate  COg,  HgO,  and  urea.  But  the  distinc- 
tive duty  of  the  lungs  is  the  elimination  of  COg,  that  of 
the  kidneys  urea,  and  that  of  the  skin  7vater  in  such  wise 
as  to  cool  the  blood.  In  each  case  the  elimination  of 
the  other  two  products  is  subsidiary  and,  as  it  were,  ac- 
cidental. Nevertheless,  we  ought  not  to  be  surprised  to 
find  a  mingling  of  these  functions  more  and  more  as  we 
go  down  the  scale. 

COMPARATIVE    MORPHOLOGY     AND    PHYSIOLOGY    OF 
THE    SKIN. 

Genera  Remarks. — i.  If  the  skin  be  dry,  harsh, 
and  without  glands,  it  is  a  sign  either  that  cooling  of 
the  blood  is  not  necessary,  as  in  cold-blooded  animals, 

*  Nature,  1,  257,  1894. 


KATABOLISM.  4-50 

or  else  that  this  function  is  performed  in  some  other  way. 
On  the  other  hand,  if  the  skin  be  soft,  moist,  and  mu- 
cous, it  IS  a  sign  that  it  is  very  active,  and  performs 
many  functions  which  have  yet  been  but  partially  differ- 
entiated and  relegated  to  an  infolded  surface. 

2.  The  function  of  cooling  the  blood  by  evapora- 
tion can  only  e.xist  in  air-breathing  and  land-inhabiting 
animals. 

Mammals. — The  structure  and  function  of  the  skin 
in  mammals  are  similar  to  the  same  in  man;  yet  there 
are  many  mammals  that  are  nonsw^eating.  Such,  in 
many  cases  at  least,  are  panting  animals.  The  most 
familiar  illustration  of  this  is  the  dog.  The  dog  is  2ivery 
hot-blooded  animal,  and  yet  in  it  there  is  no  visible 
sweat,  although  there  is,  of  course,  exhalation.  The 
cooling  of  the  blood  is  largely  through  the  lungs  by  pant- 
ing. A  dog  pants  not  because  he  is  tired  and  wants 
more  o.xygen,  but  because  he  is  hot.  By  panting  he  fans 
his  lungs. 

Birds  are  also  very  hot-blooded,  and  in  hot  weather 
they  also  supplement  the  cooling  of  the  blood  through 
the  skin  by  panting. 

Reptiles  have  dry,  scaly,  inactive  skin.  But  little  or 
no  cooling  of  the  blood  is  required  in  them,  because  the 
internal  fires  burn  low;  they  are  cold-blooded. 

Amphibians  have  the  extreme  opposite  condition. 
They  have  soft,  moist,  mucous,  and  very  active  skins, 
not,  however,  because  they  require  cooling  of  the  blood, 
for  they  are  cold-blooded  and  live  mostly  in  the  water, 
but  because  many  other  functions  are  to  some  extent 
performed  by  the  skin — for  example,  respiration  and 
even  the  absorption  of  food. 

Fishes  live  in  water,  and  there  can  be  no  evapora- 
tion from  the  skin  ;  also,  like  amphibians,  the  skin  is  soft 
and  slimy  and  active. 


440   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

Insects  are  more  or  less  warm-blooded  and  require 
blood  cooling,  yet  they  are  covered  with  a  dry,  hard, 
skeletal  coat,  which  cuts  off  entirely  any  evaporation. 
The  function  of  blood  cooling  is  undoubtedly  performed 
by  the  tracheae,  which  are  admirably  adapted  for  this 
purpose. 

Crustaceans  are  similarly  covered  and  have  no  air 
tubes,  for  they  live  in  water,  but  they  are  again  cold- 
blooded. 

Mollusca,  again,  go  to  the  other  extreme.  The  skin 
of  these  is  soft,  mucous,  and  very  active,  and  doubtless 
performs  many  functions  and  supplements  many  other 
organs. 

In  Echinoderms,  again,  we  find  the  hard  shell  and 
inactive  surface. 

In  Coelenterates  we  return  again  to  the  soft  and 
active  condition,  which  is  found  also  in  most  Protozoa. 


SECTION  V. 
The  Live?-  and  its  Fiinction. 

We  have  now  treated  of  the  eliminative  organs  con- 
nected with  anabolism  (secretory)  and  those  connected 
with  katabolisvi  (excretory)  ;  but  there  are  still  some 
whose  functions  are  of  a  mixed  character,  and  therefore 
put  off  to  the  last.     Chief  among  these  is  the  liver. 

1.  The  Organ:  Position  and  Structure.— The 
liver,  together  with  the  stomach  and  the  spleen,  fill  up 
the  concavity  of  the  diaphragm,  the  liver  being  chiefly 
on  the  right  and  the  stomach  and  spleen  on  the  left. 
Its  color,  shape,  and  size  are  well  known,  and  need  not 
detain  us,  as  they  have  no  special  relation  to  function. 
In  structure  it  differs  from  other  glands  in  not  being  en- 
tirely or  chiefly  an  aggregate  of  excretory  tubes.     On 


KATABOLISM. 


44' 


the  contrary,  it  consists  mainly  of  certain  peculiar,  very 
solid,  nucleated  cells  called  liver  cells  (Fig.  305). 

Four  Systems  of  Tubes. — An  eliminating  organ 
or  gland  has  usually  three  systems  of  tubes  or  vessels: 

(1)  The  artery  and  its  branches,  carrymg  blood  to  the 
work.  (2)  The  vein  and  its  branches,  carrying  back  the 
blood  when  the  work  is  done  ;  these  two  connect 
through  the  capillaries.  (3)  A  system  of  excretory  or 
secretory  tubes,  carrying  away  the  product  of  manufac- 
ture. But  the  liver  hdiS  four  systems  of  tubes  ramifying 
through  its  mass  of  liver  cells  :   (i)  The  hepatic  artery, 

(2)  the  hepatic  vein,  (3)  \.\\t.  portal  vein,  and  (4)  the  biliary 
ducts.     The    one   which 

is  peculiar  is  the  portal 
vein.  We  have  already 
(page  328)  drawn  atten- 
tion to  the  peculiar- 
ity of  this  vein.  It  is 
the  vein  corresponding 
to  the  mesenteric  and 
splenic  arteries.  But,  in- 
stead of  emptying  its 
blood,    like    all    normal 

veins,  into  the  vena  cava,      Fig.    -^o^.  — .Microscopic  structure  of  the 
,  11-  liver :    /c,   liver  cells ;   etc,  epithelial 

close  at  hand,  it  goes  to  ceils;  bd,  biliary  duct. 

the  liver,  to  be  ramified 

by  capillaries  through  its  substance,  and  only  after  doing 
so  carries  its  blood  to  the  hepatic  vein  and  thence  to  the 
vena  cava  and  the  general  circulation.  So  that  instead 
of  two  systems  of  vascular  pipes,  arteries,  and  veins, 
connecting  with  one  another  through  the  capillaries,  we 
have  three  systems  of  such  pipes — viz.,  hepatic  artery, 
portal  vein,  and  hepatic  vein — all  connected  continu- 
ously by  capillary  circulation,  so  that  water  injected 
into  any  one  of  these  trunks  will   flow  out  of  the  other 


442 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANLMALS. 


two.  Besides  these  tubes,  of  course,  there  are  the  bile 
ducts  (Fig.  306) ;  but  these,  like  all  excretory  tubes,  end 
in  blind  extremities  (Fig.  305).  As  we  shall  see  pres- 
ently, the  products  of  the  liver  are  bile  and  sugar.  Of 
these,  the  bile  is  carried  away  by  the  biliary  ducts  into 


Fig.  306. — Diagram  showing  the  distribution  of  the  blood  through  the  liver. 
The  arrows  show  the  direction  of  the  current. 


the  intestines,  while  the  sugar  is  delivered  to  the  blood 
and  carried  into  the  general  circulation  by  the  hepatic 
veifi. 

2.  Function  :  Two  General  Kinds  of  Glands. — 
Glands  are  of  two  general  kinds — ducted  and  ductless. 
The  one  delivers  its  products  by  a  duct  to  the  surface, 
the  other  delivers  its  product  directly  to  the  blood  with- 
out a  duct.  We  have  already  seen  two  examples  of 
these  in  the  lymphatic  glands  and  the  spleen,  and  have 
stated  (page  324)   that  the   pancreas  is  also  partly  so. 


KATABOLISM. 


443 


But  there  are  several  others — viz.,  the  thymus,  the  thy- 
roid, the  suprarenal,  etc.  Their  functions  are  very  ob- 
scure, but  very  important.  Only  recently  has  attention 
been  strongly  drawn  to  them.  Their  products  have  been 
called  internal  secretions. 

Again,  the  ducted  glands,  as  already  explained,  are  of 
two  kinds — secretory  and  excretory.  The  one  manu- 
factures useful  products  out  of  the  blood,  the  other 
eliminates  hurtful  products  of  katabolism  from  the 
blood. 

Now,  the  liver  belongs  to  all  three  kinds.  It  manu- 
factures two  things — viz.,  bile  and  sugar.  As  a  ducted 
gland  it  delivers  its  bile  by  a  duct  into  the  intestines. 
As  a  ductless  gland  it  delivers  its  sugar  directly  to  the 
blood.  Again,  the  bile  is  both  a  secretion  used  in  the 
digestive  process  and  an  excretion,  purifying  the  blood 
of  the  hurtful  katabolic  products. 

Therefore  the  function  of  the  liver  is  threefold:  (i) 
The  manufacture  of  sugar,  (2)  the  manufacture  of  bile 
as  a  secretion,  and  (3)  the  elimination  of  bile  as  an  excre- 
tion. This  last  is  probably  connected  with  the  destruc- 
tion of  the  red  blood-globules  in  the  liver.  We  have 
already  discussed  the  bile  as  a  digestive  secretion.  We 
have  also  already  spoken  of  the  liver  as  the  cemetery  of 
red  globules,  and  said  all  that  was  necessary  in  the  very 
imperfect  state  of  knowledge  on  this  subject.  All  that 
remains  is  the  discussion  of  the  liver  as  a  manufactory  of 
sugar.  This  introduces  the  very  important  subject  of 
glycogeny. 

Glycogeny  and  its  Relation  to  Vital  Force  and 
Vital  Heat.  — If  we  examine  the  blood  of  the  hepatic 
vein  we  always  find  a  notable  quantity  of  sugar. 
Whence  comes  it  ?  It  is  at  first  natural  to  suppose  that 
it  comes  from  the  digested  food,  the  sugar  of  which,  we 
have  already  seen,  is  taken  up  by  the  capillaries  of  the 


444 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


portal  vein  and  carried  to  the  liver.  But,  no.  It  may  be 
shown  to  be  formed  in  the  liver  itself.  The  following 
e.xperiments  show  it : 

1.  An  animal  may  be  fed  on  flesh  alone  (which  we 
know  contains  no  sugar-making  substance)  for  weeks 
continuously,  and  yet  sugar  will  be  found  in  the  hepatic 
vein.  Or  else  an  animal  may  be  starved  for  weeks,  and 
still  sugar  will  be  found  in  the  hepatic  vein.  Evidently 
it  must  be  made  in  the  liver. 

2.  Let  the  liver  be  taken  from  a  recently  dead  animal 
and  laid  on  the  table.  If  now  water  be  injected  into 
any  one  of  the  trunk  vessels,  the  liquid  running  out  of 
the  other  two,  if  tested,  will,  of  course,  show  sugar  ;  con- 
tinue the  current  until  only  pure  water  runs  out,  pure 
both  of  blood  and  of  sugar.  Let  the  liver  stand  a  while 
— say  an  hour.  If  now  the  current  be  started  again,  the 
issuing  water  will  again  show  sugar.  This  may  be  re- 
peated many  times  with  similar  result  until  finally  no 
sugar  comes  out.  The  material  out  of  which  sugar  is 
made  is  exhausted.  The  sugar,  therefore,  is  evidently 
made  from  some  insoluble  or  nearly  insoluble  substance, 
which  is  continually  being  changed  into  soluble  sugar 
and  washed  out. 

3.  If  a  liver  be  kept  a  considerable  time  the  sugar 
accumulates  until  the  liver  becomes  perceptibly  sweetish 
to  the  taste,  even  when  cooked. 

Now,  the  substance  from  which  sugar  is  formed  has 
been  isolated  and  its  properties  and  composition  deter- 
mined. It  is  a  colorless,  tasteless,  odorless,  nearly  in- 
soluble substance,  having  the  composition  of  starch — 
CgHiyOg.  It  is  indeed  a  kind  of  animal  starch.  It  exists 
in  the  liver  in  large  quantities — two  per  cent,  seven  per 
cent,  and  even  fifteen  per  cent  of  the  w^hole  weight. 
Like  starch  or  dextrine,  but  more  readily  than  either,  it 
is  changed  into  sugar  by  enzymes.     There  is  an  enzyme 


KATABOLISM.  ^- 

for  this  purpose  in  the  liver  cells.  The  process  of 
change  is,  of  course,  a  hydration,  exactly  like  that  which 
saccharizes  starch — viz.,  QHiuOj  +  HgO  =  QH,oOe. 
This  substance  is  called  glycogen,  or  the  sugar-maker. 
It  has  the  same  equivalent  composition  as  starch  and  dex- 
trine, but  almost  certamly  a  different  molecular  structure, 
and  therefore  slightly  different  properties.  For  exam- 
ple, it  is  more  easily  changed  into  sugar,  and  it  gives  a 
different  reaction  with  iodine.  The  sugar  formed  from 
it,  though  it  has  the  same  equivalent  composition  as 
glucose  or  intestinal  sugar,  has  also  probably  a  differ- 
ent molecular  structure,  and  therefore  also  slightly  dif- 
erent  properties.  It  is  apparently  more  easily  oxidized 
into  COg  and  HgO.  It  is  better,  therefore,  to  call  it 
liver  sugar  or  hepatose. 

The  formation  of  this  sugar  is  a  pure  chemical  not  a 
vital  process,  for  it  takes  place  in  the  dead  as  well  as  in 
the  living  liver.  The  true  vital  process  is  the  formation 
of  the  glycogen,  not  the  sugar. 

Such  are  the  undoubted  facts.  But  the  question  oc- 
curs. Whence  comes  the  glycogen  and  what  is  its  pur- 
pose ?  There  are  certain  other  facts  which  throw  light 
on  this  question. 

1.  The  amount  of  amyloid  food — say,  potatoes  and 
rice — which  is  consumed  by  a  man  in  a  day,  or  even  at  a 
single  meal,  may  be  two  pounds.  The  whole  of  this  is 
changed  into  sugar,  and,  if  carried  at  once  into  the  blood, 
would  make  that  liquid  as  sweet  as  sirup.  But,  on  the 
contrary,  the  quantity  of  sugar  in  the  general  circula- 
tion is  very  small — only  a  trace,     ^^'hat  becomes  of  it  ? 

2.  It  will  be  remembered  that  the  whole  of  the  sugar 
is  taken  up  by  the  capillaries  of  the  portal  vein,  and  car- 
ried to  be  distributed  through  the  liver  before  reaching 
the  general  circulation. 

Now,  putting  these  facts  together,  it  is  evident  that 


446   PHYSIOLOGY  AND    MORPHOLOGY    OF    ANIMALS. 

the  large  quantity  of  sugar  carried  by  the  portal  vein  to 
the  liver  is  detained  there  by  dehydration  into  the  insolu- 
ble form  of  glycogen,  and  then  slowly  and  continuously 
rehydrated  into  the  soluble  form  of  liver  sugar,  and  de- 
livered little  by  little  to  the  blood  as  the  necessities  of 
combustion  require. 

Observe  the  double  change.  The  sugar  from  the  in- 
testines is  ^/^'hydrated  only  to  be  /rhydrated.  The  rea- 
son is  twofold  :  (i)  It  must  be  stored  2,x\.di  delivered  little 
by  little,  because  sugar  in  the  blood  in  large  quantity  is 
hurtful.  Among  other  hurtful  effects,  see  the  cataract 
and  blindness  of  diabetic  patients. 

(2)  Liver  sugar  is  a  more  easily  oxidizable  sugar,  a 
more  combustible  fuel,  than  glucose.  Glucose  will  cir- 
culate in  the  blood  for  a  long  time  until  it  is  finally  ex- 
creted through  the  kidneys  in  diabetic  patients.  But 
liver  sugar  seems  to  burn  up  almost  as  soon  as  it  touches 
the  oxidized  blood. 

We  have,  then,  one  source  of  glycogen — viz.,  the  glu- 
cose taken  up  from  the  intestines  and  dehydrated  in  the 
liver;  but  this  can  not  be  the  only  source,  for  flesh-fed 
animals  also  make  liver  sugar.  Therefore  glycogen 
must  be  made  from  albuminoids  also.  How  it  is  made 
is  more  uncertain.  I  believe  it  is  made  as  follows:* 
Remember  that  albuminoid  food  excess  is  split  into  a 
combustible  carbohydrate  part  and  a  nitrogenous  incom- 
bustible part ;  the  one  eliminated  by  the  lungs,  the  other 
by  the  kidneys.  Now  I  believe  that  the  place  of  this 
splitting  is  the  liver,  the  combustible  carbohydrate 
formed  \s  glycogen,  and  the  incombustible  part  is  urea.f 

But  this  is  not  yet  enough.     There  must  be  still  a 


*  See  writer's  views,  Am.  jnur.,  xv,  (jg,  1878,  and  xi.x,  25, 
1880. 

f  Urea  is  proved  to  be  formed  in  the  liver  (Nature,  Ivii,  395, 
1S98). 


KATABOLISM. 


447 


third  source,  for  starving  animals  still  continue  to  make 
liver  sugar.  Glycogen  must  also  be  made  from  waste 
tissue.  Remember,  again,  that  waste  also  is  split  some- 
where into  a  combustible  and  an  incombustible  part,  the 
one  eliminated  by  the  lungs  as  CO2  and  HgO,  the  other 
by  the  kidneys  as  urea.  Now,  again,  is  it  not  probable 
that  l\\t place  of  splitting  is  the  liver,  and  the  combusti- 
ble portion  is  glycogen  ?  The  splitting  has  long  been 
known.  My  view  is  that  the  place  is  the  liver,  the  com- 
bustible product  glycogen,  and  the  incombustible  iirea. 

Therefore,  according  to  my  view,  there  are  three 
sources  of  glycogen  :  (i)  The  whole  of  the  amyloids  de- 
hydrated and  detained  in  the  liver,  (2)  the  combustible 
part  of  the  albuminoid  food  excess,  and  (3)  the  combus- 
tible part  of  the  waste  tissue.  But  since  in  adults  waste 
is  equal  to  repair,  this  is  equivalent  to  the  whole  of  the 
amyloids  and  the  whole  of  the  combustible  part  of  the 
albuminoid  food,  or  the  whole  combustible  food. 

But  there  are  also  exactly  the  same  three  sources  of 
vital  force.  Therefore  the  whole  purpose  of  this  function 
of  the  liver  is  the  preparation  of  fuel,  and  the  only  fuel  used 
in  the  animal  body  is  glycogen.  The  liver  prepares  the  fuel, 
the  lung  burns  it,  the  kidney  removes  the  incombustible 
residue,  or  ash.  The  only  food  not  taken  into  account 
here  is  the  fats.  How  this  is  burned,  whether  directly, 
or  whether  it  also,  as  is  most  probable,*  is  changed  into 
glycogen,  is  not  certainly  known. f 

Strong  confirmation  of  this  view,  so  far  as  waste  is 
concerned,  is  brought  out  by  some  experiments  of 
Schiff.  \  If  the  trunk  vessels  of  the  liver  of  a  dog  (Fig. 
306,  page  442)  be  ligated  so  that  the  blood  can  not  trav- 

*  Chittenden,  Nature,  Iv,  303,  1897. 

f  Berthelot,  Rev.  Sci.,  viii,  129,  1897.  Chittenden,  Sci.,  v, 
517,  1897. 

%  Arch,  des  Sci.,  Iviii,  293,  March,  1877. 
30 


448    PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 

erse  that  organ,  the  animal  speedily  falls  into  deep 
coma  and  dies  in  a  halt  hour.  Frogs  bear  well  the  liga- 
tion of  the  liver,  because  katabolic  processes  are  very 
slow  in  these  cold-blooded  animals;  but  if  a  few  drops 
of  the  blood  of  a  dog  dead  of  ligated  liver  be  injected 
into  a  frog's  veins  it  speedily  dies  if  its  liver  be  ligated j 
but  if  its  liver  be  not  ligated  it  is  not  har fried  at  all.  Evi- 
dently, therefore,  there  is  formed  by  katabolism  a  viru- 
lently poisonous  substance,  which  is  decomposed  and  ren- 
dered innocuous  in  the  liver.  This  is  certain.  Now,  my 
view  is  that  the  manner  in  which  this  substance  is  de- 
composed is  by  splitting  into  glycogen  and  urea. 

The  exact  process  of  change  of  waste  and  of  albu- 
minoids generally  in  katabolism  is  very  obscure  and  im- 
perfectly known.  Probably  it  is  complex  and  contains 
many  steps.     But  the  poisonous   results  of  albuminoid 


Animal  Life 


Fig.  307. — Diarjram  i.lus.r.t^n^j  the  r;cnerr.'.ion  of  animal  heat  and 
animal  force,  and  the  products  formed  in  the  process. 


descensive  change  are  called  in  a  general  way  leiico- 
maines.  Now,  the  propositi'n  is  that  leucomaines  are 
decomposed  in  the  liver  in  the  manner  already  explained. 
After  this  explanation  we  now  repeat  the  diagram  on 
page  422,  with  some  additions,  and  are  prepared  to  ex- 
plain it  more  fully  (Fig.  307). 

This  figure,  in  addition  to  the  general  process  by 
which  vital  force  is  generated,  gives  also  some  details: 
(i)  that  amyloids,  and  probably  fats  also,  are  changed 


KATABOLISM. 


449 


into  glycogen  before  turning  into  COg ;  (2)  that  albu- 
minoid excess  and  waste  are  changed  into  leucomaines 
before  splitting,  and,  when  split,  glycogen  is  the  com- 
bustible result. 

If  this  view  be  true,  then  glycogeny  is  indeed  a  most 
fundamental  function,  and  its  failure  must  sap  the  vital- 
ity of  the  body  in  a  marked  degree.  I  believe  that  this 
is  shown  in  glycosuria  or  diabetes. 

Cause  of  Diabetes. — This  very  grave  and  obscure 
disease,  in  which  sugar  in  large  quantities  is  excreted  by 
the  kidneys,  is  marked  by  extreme  failure  of  vitality. 
At  first  it  was  supposed  that  the  kidney  was  at  fault, 
but,  on  the  contrary,  it  does  all  it  can  to  help  the  patient. 
Sugar  in  large  quantity  in  the  blood  is  hurtful.  The 
kidneys  remove  it.  Then  it  was  supposed  that  the  lungs 
were  at  fault.  The  lungs,  it  was  said,  failed  to  take  in 
oxygen  enough  to  burn  up  the  sugar,  and  therefore  it 
must  be  excreted  by  the  kidney.  But  not  so,  for  in 
these  cases  the  blood  seems  to  be  sufficiently  oxidized. 
Then  it  was  supposed  that  the  fault  lay  in  the  liver, 
which  was  thought  to  make  too  much  sugar,  more  than 
the  lungs  could  burn.  According  to  my  view,  the  liver 
is  indeed  in  fault,  but  not  in  that  way.  Not  by  too 
much  sugar-making,  but  by  too  little glycogen-making.  The 
sugar  from  the  intestines  is  not  arrested  and  dehy- 
drated in  the  liver,  but  passes  right  through  and  floods 
the  general  circulation,  and  therefore  must  be  removed 
by  the  kidneys.  Probably  also  the  leucomaines  are  not 
split  as  promptly  as  they  ought  to  be,  and  remain  to 
poison  the  blood.  There  is  therefore  an  inadequate 
supply  of  liver  sugar,  which  is  the  necessary  fuel  of  the 
body.  It  is  easily  seen,  then,  why  this  disease  is  char- 
acterized by  low  vitality. 

Recently  another  cause  has  been  assigned  for  this 
disease — a  cause  very  important  in  relation  to  the  whole 


450 


PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


subject  of  glycogeny — by  Chavaux  and  Lepine.*  Ac- 
cording to  these  authors,  the  pancreas,  like  the  liver,  is 
both  a  ducted  and  a  ductless  gland.  As  a  ducted  gland 
it  pours  its  digestive  secretion  into  the  intestines;  as  a 
ductless  gland  it  pours  into  the  blood  a  peculiar  fer- 
ment, which  has  the  property  of  changing  liver  sugar  in 
such  wise  that  it  is  more  readily  combustible,  and  which 
is  therefore  called  glycolytic  ferment.  With  the  failure 
of  this  function  of  the  pancreas  the  sugar  is  not  burned, 
and  must  therefore  be  excreted  by  the  kidneys. 

It  seems  almost  certain,  then,  that  the  cause  of  dia- 
betes is  either  the  failure  of  the  liver  to  form  glycogen 
(by  dehydration  and  arrestation  of  sugar  in  the  case  of 
amyloids,  and  by  splitting  in  the  case  of  albuminoids), 
or  else  it  is  the  failure  of  the  pancreas  to  secrete  a 
glycolytic  ferment  necessary  for  combustion  of  sugar 
in  the  blood. 

COMPARATIVE    MORPHOLOGY    AND    PHYSIOLOGY    OF 
THE    LIVER. 

We  have  already  spoken  briefly  of  the  liver  in  con- 
nection with  the  digestive  organs  and  functions  of  dif- 
ferent animals,  and  little  more  need  be  said  here.  Its 
position,  shape,  and  even  color  is  similar  in  all  verte- 
brates, and  its  function  is  doubtless  the  same  in  all.  It 
remains  a  very  large  and  important  organ  in  inverte- 
brates even  down  very  low  in  the  scale.  It  is  nearly 
always  distinguishable  by  its  dark  color. 

As  to  glycogeny,  this  function  is  still  performed  by 
the  liver,  yet  glycogen  is  found  somewhat  widely  dif- 
fused in  the  tissues  of  many  invertebrates,  especially 
mollusks.  It  is  significant  that  the  same  is  true  of  the 
embryos  of  higher  animals.     This  is  exactly  in  accord- 


*  Rev.  Sci.,  li,  376,  1893. 


KATABOLISM. 


45' 


ance  with  the  law  of  differentiation,  speciaUzation,  and 
localization  of  functions  as  we  rise  in  the  scale.  Both 
in  the  embryo-nic  series  and  in  the  taxonomic  series 
functions  are  at  first  performed  everywhere  and  by  all 
the  tissues  ;  and  as  we  rise  in  the  scale  they  are  more 
and  more  separated,  localized,  and  perfected.  This  one 
has  been  gradually  more  and  more  localized  in  the  liver. 


CHAPTER   V. 

TEGUMENTARY    ORGANS SKIN    STRUCTURES. 

SECTION    I. 
Vertebrates. 

It  will  be  remembered  that  the  whole  surface,  both 
external  and  internal  (by  infolding  of  the  external),  is 
covered  with  a  pavement  of  living  nucleated  cells,  and 
that  all  exchange  by  absorption  or  elimination  is  by  the 
agency  of  these  cells.  When  these  are  very  active  the 
surface  is  soft  and  covered  with  a  slimy  mucus  by  the 
continual  decay  and  solution  of  dead  cells,  and  is 
called  epithelium.  When  less  active  the  old  cells  accumu- 
late in  many  layers  and  dry  up,  mummify,  and  flatten, 
and  finally  pass  away  as  scales.  This  is  called  the 
epidermis. 

The  epiderm,  therefore,  is  said  to  consist  of  two 
layers — viz.,  a  layer  of  living  nucleated  cells  in  direct 
contact  with  the  dermis  and  called  the  mucous  or  Mal- 
pighian  layer,  and  a  layer  of  accumulated  cells  in  all 
stages  of  dying  and  flattening  and  mummification,  and 
called  the  ctiticular  layer.  The  color  of  the  skin,  whether 
blond  or  brunette,  or  brown  or  black,  is  determined  by 
the  amount  of  pigment  in  the  mucous  layer.  The 
cuticular  layer,  in  the  act  of  drying  up,  may  harden  in 
various  degrees. 

For  example,  if  the  epithelial  cells  only  mummify 
they  form  the  ordinary  cuticle.  If  they  simply  harden, 
452 


TEGUMENTARV    ORGANS— SKIN    STRUCTURES. 


453 


they  form  horn,  as  in  hair,  nails,  claws,  hoofs,  horns, 
feathers,  scales,  whalebone,  etc.  If  they  calcify  or  ossify 
by  deposit  of  lime  car- 
bonate or  phosphate, 
then  they  form  bony 
scales,  scutes,  shell, 
etc. 

Importance  in 
Classification.  —  As 
the  skin  comes  directly 
in  contact  with  and  is 
modified  by  the  envi- 
ronment, these  struc- 
tures are  very  charac- 
teristic of  the  various 
classes,  orders,  and 
families  of  animals, 
and  are  therefore  very 
important  in  classifi- 
cation. We  have  al- 
ready shown  that  teeth 
are  skin  structures, 
and  their  importance 
in  classification  is  uni- 
versally acknowl- 
edged. The  structures 
about  to  be  described 
are  hardly  less  so.  We 
give  only  the  most  im- 
portant. 

Hair. — True  hair  is  entirely  characteristic  of  mammals. 
It  is  always  formed  in  a  follicle  or  infolded  socket,  in 
which  the  epithelial  cells  multiply  more  rapidly  than 
elsewhere  and  at  the  same  time  harden  to  the  condition 
of  horn  (Fig.  308).     Hairs,  therefore,  grow  by  successive 


■o'-'O  K'^^-^. 


Fig.  308. — Section  of  skin  shewing;  hair 
follicle  :  pap,  papilla  ;  ml,  .Malpighian 
layer  ;  rsh,  root  sheath  ;  t/,  dermis  ;  sgl, 
sebaceous  gland  ;  f ,  cuticle  ;  cort,  corti- 
cal, and  med.,  medullary  part  of  the  hair. 


454 


PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 


additions  to  the  bottom,  pushing  outward,  not  by  addi- 
tions to  the  top  Uke  a  tree.  The  follicle  in  which  it  is 
formed  acts  as  a  mold,  determining  its  shape.  The 
quills  of  the  porcupine  are  only  large  hairs. 

Nails. — If  we  tear  off  a  finger  nail,  we  find  beneath 
an  exquisitely  tender  surface  of  dermis  covered  with  a 
layer  of  very  active  epithelium.  At  the  base  of  the  nail 
is  a  pocketlike  infolding,  where  the  epithelium  is  espe- 
cially active.  Over  the  whole  surface  the  epithelial  cells 
harden  into  horn,  but  this  process  is  especially  active 
in  the  pocket  (Fig.  309).  Therefore  the  nail,  by  its 
more  rapid  formation  in  the  pocket  at  its  base,  is  pushed 
forward  continuously,  and,  if  not  worn  away  or  pared 
away,  will  grow   indefinitely.     In   some  countries,  as  in 


Fig.  309. — Section  of  the  end  of  the 
finger,  showing  how  the  nail  is 
formed  :  c,  cuticle  ;  ml,  Malpighi- 
an  layer ;  hc^  the  horny  cuticle  ; 
^,  the  bone. 


Fig.  310. — Section  through  terminal 
joint :  clcr,  claw  core  ;  he,  horny 
cuticle  ;  ml,  Malpighian  layer ; 
c,  unhardened  cuticle. 


Japan,  they  are  sometimes  protected  and  become   enor- 
mously long,  as  a  badge  of  a  leisure  class. 

Claws. — These  differ  from  nails  only  in  the  fact  that 
they  grow  on  all  sides  instead  of  one  side  of  a  peculiarly 
shaped  terminal  joint  of  a  finger  or  toe.  The  bone  of 
the  terminal  joint  is  the  claw  core  and  determines  the 
shape  of  the  claw.  The  core  is  covered  with  dermis  and 
with  an  active  layer   of  epithelium,  which   hardens  into 


TEGUMENTARY    (ORGANS-SKIN    STRUCTURES. 


455 


horn.  At  the  base  there  is  an  infolded  pocket  in  which 
growth  is  more  active,  so  that  the  claw  is  pushed  for- 
ward in  proportion  as  it  is  worn  off  by  use  (Fig.  310). 

Hoofs. — These,  again,  differ  from  claws  only  by  the 
size  and  shape  of  the  terminal  joint  on  which  thev  are 
molded.  This  terminal  joint  is 
the  hoof  core  or  coffin  bone.  We 
have  here  the  same  pocketlike 
infolding,  and  the  forward-push- 
ing growth  in  proportion  to 
wear  (Fig.  311).  If  wearing  is 
prevented  by  shoeing  they  must 
be  trimmed  from  time  to  time. 

The  horny  armature  of  the 
terminal  joints  of  the  toes  is 
characteristic  of  land  vertebrates 
— i.  e.,  reptiles,  birds,  and  mam- 
mals. They  are  absent  even  in 
amphibians  on  account  of  their 
greater  alliance  to  fishes.  Furthermore,  mammals,  by 
the  character  of  this  armature,  are  divided  into  ungulates 
(hoofed)  and  unguiculates  (clawed). 

The  original  generalized  form  of  this  armature  was 
apparently  of  flattened  shape  on  the  dorsal  side  of  the 
toe  somewhat  like  that  of  man.  This  was  the  case  in 
some  of  the  earliest  mammals,  as,  for  example,  the 
Phenacodus,  and  has  been  retained  by  apes  and  by  man. 
From  this  generalized  form  have  been  differentiated 
claws  on  the  one  hand  and  hoofs  on  the  other. 

Horns. — These  are  almost  characteristic  of  rumi- 
nants. Paired  horns  on  frontal  bones  are  entirely  so  at 
present,  although  not  so  in  early  geological  times.  Again, 
frontal  horns  are  of  two  kir.ds — viz.,  solid  horns,  as  in 
the  Cervidce  (deer  family),  and  hollow  horns,  as  in  the 
Bovidce  and  Ovidie — e.  g.,  o.\,  sheep,  goats,  etc. 


Fig.  311. — The  joints  of  the 
toe  of  a  horse.  The  parts 
are  similar  to  the  last  two 
figures. 


456 


PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


Fig.  312. — Section  throug-h 
a  cow's  horn  :  c,  cuticle ; 
hc^  horny  cuticle  ;  w//,  Mal- 
pighian  layer  ;  //cr,  horn 
core ;  d^  dermis. 


In  hollow  horns  (Fig.  312)  we  have  first  a  conical 
projection  of  the  frontal  bone,  covered  as  usual  with 
dermis,  and  this  in  its  turn  with  active  epithelium  hard- 
ening into  horn.  About  the  base 
there  is,  as  usual,  an  infolding  in 
which  the  growth  of  horn  is  espe- 
cially rapid.  Successive  layers 
are  formed  one  within  another 
precisely  as  in  claws.  The  hol- 
low horns  are  permanent. 

Solid  horns,  as  in  the  deer,  elk, 
etc.,  are  bosses  which  grow  out 
from  the  two  frontal  bones  with 
great  rapidity,  and  are  at  first 
covered  with  skin  and  fine  hair 
(so-called  velvet).  In  this  condition  it  is  very  vascular. 
In  a  little  while — a  month  or  so — the  blood  gradually 
withdraws,  the  skin  dies  and  is  rubbed  off,  and  the  antler 
is  really  dead  though  firmly  attached  to  the  skull.  At 
the  end  of  the  year  a  separation  takes  place  at  the 
skull,  the  antler  drops,  and  the  skin  of  the  skull  grows 
over  and  covers  the  wound.  Soon  growth  begins  again 
at  the  same  place  and  a  new  pair  of  antlers  is  formed,  to 
pass  again  through  the  same  annual  cycle  of  changes. 

In  comparing  the  antler  of  a  deer  with  the  horn  of 
an  ox  or  sheep  it  is  evident  that  the  mature  antler  of 
the  former  corresponds  with  the  horn  core  of  the  latter, 
while  the  horny  exterior  of  the  latter  corresponds  to  the 
skin  and  velvet  of  the  former. 

Thus,  then,  the  ruminants  are  divided  into  two  groups, 
the  hollow-horned  and  the  solid-horned.  The  horns  of 
the  former  are  permanent,  those  of  the  latter  are  decidu- 
ous.    The  two  grade  into  one  another  in  the  antelopes. 

Feathers. — This  most  wonderful  of  all  skin  struc- 
tures is  whollv  characteristic  of  birds. 


TEGUMENTARY   ORGANS— SKIN    STRUCTURES. 


457 


Structure. — An  ordinary  quill  feather  is  a  marvel 
of  lightness,  strength,  and  elasticity.  The  quill  is  hollow 
— a  form  which  gives  greatest  strength  for  the  same  weight 

(pf  sk    p.v 


Fig.  313.— Section  throuj^h  the  successive  feathers  of  a  bird's  wing,  showing 
the  mode  of  overlap  :  s/i,  shaft ;  av,  anterior,  and  pv,  posterior  vane. 


A.A  ,,,)il 


of  material.  The  s/ia/l  may  be  regarded  as  a  hollow 
tube  filled  within  with  lightest  bracing,  with  the  horny 
envelope  thickest  on  the  back,  where  the  strain  comes  in 
flight.  On  each  side  of  the  shaft  is  the  7a/ie.  It  will  be 
observed  that  the  two  parts  of  the  vane  are  not  equal, 
the  backward  07'erlaJ>/>ed  pa.rt,J>  v,  being  much  the  broader. 
The  effect  of  this 
is  to  close  up  the 
feathers  into  a 
solid  plane  in  the 
downward  blow 
of  the  wing,  while 
it  opens  the  feath- 
ers and  lets  the 
air  through  in  the  sh-^ 
upstroke  (Fig. 
313).  x\gain,  the 
vane  consists  of 
barbs,  the  shafts  of 
which  are  broad 
and  therefore 
strong  in  a  verti- 
cal direction,  and 

on  these  barbs  again  are  a  vanelet  on  each  side  com- 
posed of  barbulcs,  and  finally  the  barbules  are  hooked 
together  by  little  elastic  hooks  (Fig.  314).     In  Fig.  315 


Fig.  314. — Portion  of  a  shaft  ish'^  and  vane  on 
one  side,  showing;  b)  the  barbs  and  yb' )  the 
barbules  with  their  hooks. 


458    PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 


Fig.  315. — A  magnified  view  of  one  barb  (b  b), 
with  its  barbules  (b' }  armed  with  hooks. 


we  give  a  magnified  view  of  one  barb  with  its  barbules 
and  hooks.  The  object  of  this  structure  is  to  make  an 
impermeable,  light,  and  elastic  plane.     If  these  delicate 

parts  are  disarranged 
or  ruffled,  they  are 
easily  rearranged, 
and  hooked  together 
again  by  preening. 
So  much  for  the  de- 
vice for  making  an 
impermeable  plane. 
Now  observe  how 
the  plane  is  set  on  the  fore  limb.  It  is  not  set  on  both 
sides,  but  only  on  the  backward  side  of  the  extended 
arm.  The  downward  stroke  of  the  wing  therefore  tips 
up  the  wing  plane  behind,  so  that  the  same  stroke  sus- 
tains and  also  drives  forward.  In  Fig.  316,  which  is  a 
cross  section  of  the  wing  plane,  if  i,  3  represent  the  whole 
air  pressure  in  the  down- 
stroke,  then  I,  2  will  be  ^^ 
the  force  which  sustains 
the  bird,  and  3,  2  that 
which  drives  it  forward. 
How  Formed.  — 
We  have  seen  that  all 
these  structures  thus  far 
are  formed  in  or  on  a 
mold,  which  determines  their  shape,  and  from  which  they 
are  pushed  out.  Now,  the  same  is  true  of  feathers  also. 
All  this  complex  structure  is  secreted  in  an  equally  com- 
plex mold,  from  which  it  is  necessarily  pushed  out,  and 
the  mold  corresponding  successively  destroyed. 

Gradation  to  Hairs. — We  have  described  the  quill 
feathers  of  the  wings  and  tail,  for  these  are  most  com- 
plex and  characteristic,  but  all  gradations  may  be  traced 


Fig.  316.—  Cross  section  of  the  wing 
plane  of  a  bird,  showing  its  action  in 
flying. 


TEGUMENTARV    ORGANS— SKIN    STRUCTURES. 


459 


Fig.  317. — Diagfram  of  an  ostrich  plume,  con- 
sistinj^  of  shaft  (s/i  i,  barbs,  and  barbules,  but 
no  hooks. 


through  the  body  feathers,  down,  and  plumes,  to  simple 
hairs.  The  plumes  of  the  ostrich  have  the  structure 
of  feathers,  except 
they  have  no  hook- 
lets,  and  therefore 
the  barbules  do 
not  cohere  (Fig. 
317).  Plumes  like 
those  of  the  heron, 
egret,  etc.,  have  no 
barbules;  they  are 
essentially  slender, 
branching  hairs 
(Fig.  318).  Again, 
about  the  beaks  of 
many  birds  we  find 
simple  hairs  (Fig. 
319).  This  would 
seem  to  indicate  a 
close  relation  in 
this  regard  be- 
tween birds  and 
mammals.  But 
birds  undoubtedly 
came  from  reptiles, 

and  therefore  feathers  are  probably  some  modifica- 
tion of  scales.  But  the  gradations  here  have  not  been 
found. 

Scales  are  characteristic  of  reptiles  and  fishes,  espe- 
cially the  latter. 

Fish  Scales. — We  take  these  as  the  type.  They  cover 
the  whole  body.  They  are  formed  much  like  nails — 
i.e.,  on  the  surface  of  the  mucous  layer,  and  especially 
in  pocketlike  infoldings.  The  manner  in  which  from 
those   pockets  they  grow  backward,  shingling  over  one 


Fig.  318. — Diagram  of  an  egret  plume,  consist- 
ing only  of  shaft  and  barbs. 


Fig.  319. — Hair  about  the  beak  of  a  bird,  con- 
sisting of  shaft  only. 


460  PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

another,  is  shown  in  the  figure  (320),  which  is  taken  with 
some  simplification  from  Owen. 

Classification  of  Fishes  by  Scales. — Agassiz  di- 
vided  fishes  by  the  character  of   their  scales  into  four 


Fig.    320. — Section    through   skin   of  a  fish:   d,    dermis;  tnl,  Malpighian 
layer  ;  sc,  scale,     v  From  Owen. ) 

orders — viz.,  the  Ctenoids,  the  Cycloids,  the  Ganoids,  and 
the  Placoids.  Ctenoid  scales  are  pectinate  on  their  pos- 
terior or  exposed  margin  (Fig.  321,  A).  Cycloid  scales 
are  smooth  and  rounded  on  this  margin  (Fig.  321,  B). 
Ganoid   scales   are  bony,   enameled,  and  usually  rhom- 


D 


Fig.  321.— Fish  scales,  illustrating  Agassiz's  classification. 


boidal,  close  fitting,  and  not  shingling  (Fig.  321,  C). 
Placoid  scales  are  very  small,  with  outsticking  sharp 
points  (Fig.  321,  D).     The  first   two  are  horny,  the  last 


TEGUMENTARY    ORGANS— SKIN    STRUCTURES. 


461 


two  are  bony.  The  Ctenoids  are  spine-rayed  fishes,  like 
the  perch,  etc.  ;  the  Cycloids  are  soft-rayed  fishes,  Hke 
the  cod  ;  the  Ganoids  are  sturgeons  and  bony  pikes  or 
gars;  iht  Piacoids  are  sharks,  skates,  and  rays.  If  we 
make  the  Ctenoids  and  Cycloids  subdivisions  of  teleosts  ox 
ordinary  typical  fishes,  then  the  classification  is  a  good 
one  as  far  as  it  goes,  and  has  done  good  service  in  geol- 
ogy? for  it  is  the  scales  which,  together  with  the  teeth, 
are  most  apt  to  be  preserved. 

Reptile  Scales. — We  have  taken  fish  scales  as  the  type  ; 
but  scales  are  found  also  in  reptiles,  and  even  in  mam- 
mals. In  reptiles  they  may  be  horny,  as  in  snakes  and 
lizards,  or  bony  scutes,  as  in   Crocodilia,  or  a  combina- 


C^d 


Fig.  322. — Diagram  showing-  strrcture  and  mode  of  formation  of  a  rattle- 
snake's rattle:  abc,  horny  cuticle  of  last  joints  of  tlie  vertebra  [V\\ 
a'  h'  c\  that  of  last  year  slipped  back  and  caught ;  ad"  c  ,  and 
a"  b'"  c"\  are  still  earlier  cuticles. 


tion  of  those  two,  as  in  the  shells  of  tortoises  and  tur- 
tles. Scutes  differ  from  scales  proper  in  involving  the 
dermis  as  well  as  the  epidermis. 

In  snakes  the  horny,  scaly  cuticle  is  shed  every  year, 
and  a  new  one  is  formed  by  the  mucous  layer.  This  an- 
nual skin  shedding  gives  rise  in  an  interesting  way  to 
the  rattle  of  the  rattlesnake.  The  few  last  joints  of  the 
vertebral  column  (a,  b,  c)  are  enlarged  and  consolidated 
into  an  irregular  mass,  and  covered  with  a  horny  scale 
thicker  than  elsewhere  on  the  body  (Fig.  322).  With 
the  skin  shedding  this  loosens,  and  is  shed  like  the 
rest  of  the  skin,  but,  being  elastic,  it  slips  back  and 
catches   on    the   neck   of  the   next   joint.     In   the   next 


462    PHYSIOLOGY    AND    MORPHOLOGY    OF    ANIMALS. 

year's  skin  shedding  the  horny  covering  of  this  part 
slips  back  again,  and  is  caught  in  the  same  way,  and  so 
on.  Thus  is  formed  a  series  of  molds  of  the  three  con- 
solidated joints  loosely  caught  together.  Every  year 
adds  another  to  the  chain,  and,  if  not  broken  or  worn 
off,  the  number  indicates  the  number  of  skin  sheddings, 
and  therefore  the  age  of  the  snake. 

Turtle  Shell. — Turtles  are  incased  in  an  immovable 
shell.  The  dorsal  part  is  called  the  carapace,  the  ventral 
part  tht  plastron.  Where,  then,  is  the  jointed  backbone 
characteristic  of  vertebrates  ?  Their  structure  seems  to 
violate  the  vertebrate  plan.  But  not  so.  It  is  only  an 
extreme  modification  of  that  plan,  and  is  an  admirable 
illustration  of  adaptive  modification  underlying  homology 
(page  246).  If  we  look  into  the  interior  of  the  carapace 
of  a  complete  skeleton  of  a  turtle  we  see  a  continuous 
series  of  vertebrne,  consolidated  with  one  another  and 
with  the  shell  in  the  region  of  the  trunk,  but  movable 
in  the  neck  and  tail.  The  shell  of  a  turtle  or  tortoise  is 
indeed  a  very  complex  structure,  consisting  of  three 
parts — skeletal,  dermal,  and  epidertnal.  The  spinous  pro- 
cesses of  the  vertebrae  expand  at  the  top  into  broad  flat 
plates,  which  unite  with  one  another  to  form  the  ridge  ox 
central  row  of  plates  of  the  carapace.  The  ribs  also 
expand  into  broad  plates,  which,  uniting,  form  the  slop- 
ing under-roof  on  each  side.  Then  the  dermis  over  all 
this  is  ossified  into  bony  plates,  which  unite  with  the 
skeleton  proper  to  form  the  main  part  of  the  carapace. 
Lastly,  the  epidermis  completes  the  structure  by  forming 
the  horny  covering  over  all.  This  last  is  the  so-called 
tortoise  shell  so  much  prized.  Between  this  and  the  bony 
shell,  of  course,  there  is  a  Malpighian  layer,  which  by 
transformation  forms  the  horny  layer.  Similarly  the 
plastron  is  formed  by  the  union  of  dermal  bony  plates 
with   the   ventral  ribs  (like  those   of  an  alligator),  and 


TEGUMENTARY    ORGANS— SKIN    STRTCTURES. 


463 


covered  with  epidermal  horn.  In  both  the  carapace  and 
the  plastron  the  epidermal  horny  plates  do  not  corre- 
spond with  the  bony  plates  beneath,  but  break  joints  with 
them. 

Mammalian  Shell. — At  the  present  time  shells  are 
found  only  in  the  armadillos  and  pangolins,  but  in  early 
times — Quaternary — armored  mammals  were  numerous 
and  of  great  size. 

Endoskeleton  and  Exoskeleton. — Thus,  even  in  mam- 
mals and  much  more  in  reptiles,  we  begin  to  have  the 
distinction  between  an  exterior  shell  and  interior  skele- 
ton, or  exoskeleton  and  endoskeleton.  I  n  vertebrates  the  exo- 
skeleton is  purely  protective^  but  ni  invertebrates,  which 
have  no  endoskeleton,  it  becomes  locomotive  as  well  as 
protective.     This  brings  us  naturally  to  the  invertebrates. 

SECTKJX  II. 
Invertebrates. 
ARTHROPODS. 

Insects. — Arthropods  are  all  covered  with  an  effi- 
cient exoskeleton,  both  protective  and  locomotive.  In 
insects  it  is  composed  of  chitin  (a  partly  calcified  horny 
substance).  As  this  is  usually  rigid  and  unyielding  and 
is  not  shed,  insects  can  not  grow  after  they  have  once 
put  on  this  coat  of  mail.  Therefore  the  whole  growth 
must  take  place  in  the  soft  larval  condition.  Neither 
butterflies,  nor  beetles,  nor  flies,  nor  bees  and  ants,  etc., 
grow.  They  finish  their  growth  in  the  form  of  cater- 
pillar or  grub.  Many  extremely  beautiful  and  curious 
appendages  are  found  on  insects  in  the  form  of  elab- 
orately sculptured  scales,  and  the  splendid  colors  of 
insects  are  mainly  due  to  these.  Such  are  the  scales 
which  give  color  to  butterflies,  and  the  gorgeous  iri- 
descent green-gold  hues  to  some  beetles. 
31 


464   PHYSIOLOOV    AND    MORPHOLOGY   OF   ANIMALS. 

The  Higher  Crustaceans  are  equally  incased  in 
an  unyielding  shell  more  calcified  than  that  of  insects; 
and  yet  they  gr on.'.  This  is  possible,  however,  only  by  a 
periodic  shedding  of  the  shell,  which  leaves  them  for  a 
time  almost  helpless  for  want  of  a  rigid  skeleton  until  a 
new  one  is  formed  by  deposit  of  carbonate  of  lime  in  the 
skin. 

In  lower  crustaceans  the  shell  is  chitinous,  like  that 
of  insects.  Thus  crustaceans  have  been  divided  by  their 
shell  substance  into  a  lower  group  [Ento»tostraca),  in- 
sect-shelled or  chitinous-shelled,  and  a  higher  group 
{^Malacosiraca),  mollusk-shelled  or  calcareous-shelled. 


MOLLUSKS. 

These  are  par  excelletice  shell-covered  animals,  and 
their  classification  is  largely  based  on  the  character  of 
their  shells. 

Acephala  have  two  shells,  a  right  and  a  left,  hinged 
along  the  back — bivalves.  Gastropods  have  but  one,  usu- 
ally much  coiled  shell — 
univalves.  Cephalopods, 
when  they  have  a  shell 
at  all,  are  distinguished 
by  their  many-chambered 
structure.  The  animal 
lives  only  in  the  large 
outer  chamber.  All  the 
other  chambers  are 
closed  and  filled  with 
air,  and  a  slender  mem- 
branous tube  —  the  siphicncle — runs  from  the  animal 
through  all  these  chambers,  but  not  opening  into  them 
(Fig.  325)- 

Growth  of  Shell. — The  shell  is  a  calcareous  secre- 
tion by  the  skin  of  the  mantle.     In  bivalves  (see  Figs.  280 


Fig.  323. — Surface  view  of  a  bivalve, 
showing  lines  of  growth  or  successive 
sizes  of  the  shell. 


TEGUMENTARV   ORGANS— SKIN    STRUCTURES.     465 


Fig.  324. — Section  along  the   line  a  b  oi 
previous  figure,  showing  structure. 


and  281,  pages  401  and  402)  the  mantle  covers  the  inte- 
rior of  the  shell  to  the  very  edge,  and  forms  the  shell  in 
its  epidermic  cells,  lay- 
er by  layer,  each  on  the 
inside  of  the  last,  and 
extending  a  little  be- 
yond it,  by  the  growth 
of  the  animal.  The 
successive  growths  are 
easily  seen  on  the  out- 
side of   the  shell  (Fig. 

323;  on  section.  Fig.  324).  If  a  small  object  like  a 
coin  be  slipped  between  the  mantle  and  the  shell  it  will 
soon  be  covered  by  a  secretion  from  the  mantle,  and 
finally  inclosed  in  the  thickness  of  the  shell. 

The    subdivisions  of    the  Acephala  are   seen    in    the 
shells.     The  Monomyaria  have  one  muscular  impression 

on  the  shell,  as  in 
the  oyster ;  the  Dhn- 
yaria  have  two  mus- 
cular impressions,  as 
in  the  clam  or  the 
river-mussel,  etc. 

The  gastropod 
shell  grows  in  a  sim- 
ilar   way,    and    the 
successiveshell  mar- 
gins   can    generally 
be    recognized,   and 
often  form  conspicu- 
ous   ornaments    on 
the  shell. 
Cephalopods  live,  as  already  said,  in   the   larger  outer 
chamber,  all  the  others  being  empty  and  connected  with 
one  another  in  the  living  animal  by  the  siphuncle  (Fig. 


Fig.  325.— Section  through  a  nautilus  shell: 
ch,  living  chamber ;  ch\  empty  chamber : 
si  si,  siphuncle  ;  s  s,  septa.  A  new  septum 
to  be  formed  and  an  extension  of  the  outer 
chamber  is  shown  by  the  dotted  lines. 


466  PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 

325).  As  the  animal  grows,  the  extenduig  mantle  adds 
to  the  length  and  size  of  the  outer  chamber  until  finally 
the  animal  withdraws  from  contact  with  the  last  cham- 
ber wall  and  secretes  another  partition,  and  thus  adds 
another  to  the  series  of  empty  chambers.  Cephalopods 
are  classified  into  two  groups — viz.,  the  naked,  like 
squids  and  cuttlefish,  and  the  shelled,  like  the  nautilus; 
the  former  have  two  gills  (dibranchs),  the  latter  four 
gills  (tetrabranchs). 

The  exquisite  beauty  and  variegated  iridescent  luster 
of  molluscous  shells  when  polished  is  the  result  of  the 
superposition  of  extremely  thin,  translucent  plates  and 
the  interference  of  light  thus  produced.  The  play  of 
color  becomes  more  splendid  if  the  plates  are  corru- 
gated, as  in  the  abalone  [Haliofiis). 

ECHINODERMS. 

The  shell  of  a  sea  urchin  is  a  wonderful  structure 
when  examined  under  a  microscope.  Imagine  a  calca- 
reous shell  composed  of  several  layers,  each  layer  a 
reticulation  of  calcareous  fibers,  the  several  layers  sepa- 
rated from  one  another  by  calcareous  pillars,  but  the 
openings  or  mesh  of  the  several  layers  not  coincidiiig — 
and  we  have  a  structure  which  is  a  marvel  of  lightness 
and  strength  combined  with  perfect  permeability.  This 
structure  is  easily  seen  under  the  microscope,  and  the 
skin  structures  of  echinoderms  generally  furnish  some 
of  the  most  beautiful  of  microscopic  preparations,  and 
are  very  characteristic  of  the  several  orders. 

CORALS. 

The  deposits  of  coralline  limestone  in  corals  are  some- 
what peculiar,  yet,  as  they  are  formed  by  epidermal  cells, 
like  epithelial  cells,  and  therefore  are  of  similar  origin, 


TEGUMENTARV   ORCIANS— SKIN    STRUCTURES. 


467 


•they  may  well  be  treated  under  this  head.  Everywhere 
the  deposit  is  in  the  epidermis;  for  even  the  partitions 
are  infoldings  of  the  epiderm. 


Fig.  326. — Simplified  figure  of  an  actinia. 

We  recall,  then,  the  structure  of  the  soft  polyp,  with 
its  radiating  partitions,  and  reproduce  the  figure  show- 
ing it  (Fig.  326).     Now,  in  corals,  the  structure  is  the 


Fig.  327.— Ideal  section,  vertical  and  horizontal,  showing  structure  :  /,  ten- 
tacles ;  s,  stomach  ;  //,  partitions. 

same,  except  that  deposits  of  lime  carbonate  are  formed 
in  the  lower  part— i.  e.,  in  the  foot  disk,  the  walls,  and 
the  partitions  to  more  than  midway  up  the  animal, 
but  leaving  the  upper   part   of  the   wall,  partitions,  and 


468    PHYSIOLOGY   AND    MORPHOLOGY   OF    ANIMALS. 


the  tentacles  soft  and  free  (Fig.  327).  When  the  coral  • 
teems  to  disappear,  as  in  a  cell,  it  is  simply  the  with- 
drawal of  the  soft  up- 
per parts  into  the  lower 
calcareous  part  or  the- 
ca.  In  the  dead  coral 
the  organic  matter  dis- 
appears and  leaves  the 
cuplike  theca  showing 
the    radiated    structure 

Fig    328.-^.  stonypartof  a  single  coral;       f  ^^e  animal  (Fig.  328, 
b,  section  of  same,  showing  structure.  \      tj    o       > 

a  and  b). 
Theca. — Only  recently  has  the  origin  of  the  theca 
been  well  understood.     It  was  formerly  supposed  that 
the    coral    limestone    was    deposited   in 
the  substance  of  the  tissues,  and  there- 
fore constituted  a    sort  of  endoskeleton. 
But  now  it  is  known  to  be  a  true  exo- 
skeleton    or    an    epidermal 
structure.     The   mode  of 

formation  seems  to  be  as  I'     WSSSSM^^!S^^MWm^^ 

follows:     (i)    The    basal 


Fig.  329. — Basal  plate  or  foot  disk 
of  a  larva  of  coral,  showing  the 
commencing  laniellae  of  the  theca. 
(After  Sedgwick. j 


Fig.  330.— Diagram  showing  struc- 
ture of  the  theca  ;  the  white  in 
section  is  coral  limestone  :  Bp^ 
basal  plate  ;  Tli,  theca  ;  ep,  epi- 
theca  ;  ec^  ectoderm  or  epiderm  ; 
en^  endoderm  ;  7",  tentacles ;  ae, 
oesophagus  ;  ?«,  mesentery  ;  ;«/", 
mesenteric  filaments.  (After 
Sedgwick. ) 


TEGUMENTARY   ORGANS— SKIN   STRUCTURES. 


469 


plate  forms  radiating  upgrowths  by  the  infolding  from 
below  of  the  epiderm  and  deposit  of  calcareous  matter 
in  the  folds  (Fig.  329);  (2)  the  outer  margins  of  the 
radiating  calcareous  septa  unite  to  form  the  outer  wall 
of  the  theca;  (3)  the  body  wall  by  downfolding  grows 
over  this  outer  wall,  inclosing  and  covering  it  and  add- 
ing to  it  by  calcareous  deposit,  and  thus  completing  the 
outer  surface  (Fig.  330). 

SPONGES. 

The  skeletal  deposits  in  sponges  also  are  formed  by 
the  agency  of  unmodified  cells,  and  therefore  may  be 
brought  under  this  head.  What  we  call  a  spotige  is  the 
horny  skeleton  of  the  animal  of  the  same  name,  and 
deposited  within  its  tissues  in  the  most  intricate  way. 


Fig.  331. — Shells  of  living  foraminifera  :  A,  textularia  variabilis;  B,  pene- 
roplis  planatus ;  C,  rotalia  concamerata.  (After  Williamson.)  The 
figures  are  greatly  enlarged. 


The  glass  sponge,  such  as  the  Venus's  flower-basket — 
Euplectella — with  its  beautiful  and  intricate  mesh  of 
glass  fibers,  is  similarly  produced  by  a  deposit  of  silica 
in  the  tissues  of  the  animal.  The  classification  of 
sponges  is  based  on  the  character  of  '  the  skeleton. 
There  are  horny  sponges,  glass  sponges,  and  calcareous 
sponges. 


470 


PHYSIOLOGY    AND    MORPHOLOGY    OF   ANIMALS. 


RHIZOPODS. 


Many  rhizopods  have  no  hard  parts,  but  others, 
especially  Foraminifera  and  Radiolaria,  form  calcareous 
or  siliceous  skeletons  of  exquisite  beauty  and  com- 
plexity. They  form  the  most  beautiful  objects  under 
the  microscope.  These  skeletons  are  also  very  charac- 
teristic of  the  various  orders  (Fig.  331). 


CHAPTER    VI. 

GEOGRAPHICAL    DISTRIBUTION    OF    ORGANISMS. 

This  subject  is  not  directly  connected  with  physi- 
ology, although  it  is  with  morphology,  but  its  extreme 
interest  and  importance  in  connection  with  evolution 
justifies  its  treatment  with  some  fullness. 

It  is  well  known  that  the  observant  traveler  from 
one  continent  to  another — as,  for  example,  to  take  an 
extreme  case,  from  Europe  or  the  United  States  to 
Australia — finds  all  the  animals  and  plants  entirely  dif- 
ferent from  those  he  has  been  accustomed  to  see  at 
home.  The  same  is  true  in  less  degree  in  going  from 
America  to  Europe,  or  even  from  the  Atlantic  to  the 
Pacific  side  of  our  own  continent.  Until  comparatively 
recently  the  facts  of  this  distribution  of  species  were  a 
mere  chaotic  mass  without  a  guiding  principle.  The 
theory  of  evolution  has  brought  law  and  order  into  this 
chaos.  All  we  can  do  here  is  to  give  a  bare  outline  of  the 
general  laws  of  this  distribution  and  their  explanation. 

Fauna  and  Flora. — In  popular  language  fauna 
and  flora  means  the  group  of  animals  and  plants  inhabit- 
ing any  place;  but  in  scientific  language  it  is  a  natural 
group  of  organisms  differing  from  other  natural  groups 
and  separated  from  them  by  geographical  boundaries,  or 
by  temperature  or  climate,  or  physical  conditions  of 
some  sort,  and  in  harmonic  relations  with  one  another 
and  with  the  environment. 

471 


472 


PHYSIOLOGY   AND   MORPHOLOGY   OF   ANIMALS. 


Illustrations  of  Harmonic  Relations. — The  com- 
plexity of  the  harmonic  relations  of  all  members  of  a 
group  of  organisms  is  such  that  if  one  element  is  altered 
a  wave  of  change,  often  of  the  most  unexpected  kind, 
is  propagated  through  the  group  until  a  new  harmonic 
relation  is  established.  It  was  Darwin  who  first  drew 
attention  to  the  relation  of  cats  to  the  flourishing  of 
clover.  The  fertilization  of  clover  flowers  is  dependent 
on  the  presence  of  bumble  bees,  but  the  nests  of  these  are 
destroyed  hy  field  mice,  and  field  mice  are  destroyed  by 
cats.  Professor  Morgan  somewhat  humorously  extends 
the  complex  relation  by  adding  that  cats  are  cherished  by 
old  maids.  Thus  the  presence  of  old  maids  is  favorable 
to  the  growth  of  clover.  The  most  unexpected  results 
often  come  from  interference  with  these  natural  rela- 
tions. Farmers  to  protect  crops  destroy  birds,  and  in- 
sects injurious  to  crops  increase.  Sportsmen  introduce 
English  rabbits  into  Australia  and  New  Zealand,  and 
the  governments  of  those  countries  have  spent  millions 
of  pounds  in  vain  attempts  to  destroy  them. 

In  a  word,  every  species  in  order  to  continue  to  exist 
must  be  in  harmonic  relation  with  the  environment  both 
physical  and  organic.  Now,  the  physical  environment 
consists  of  soil,  climate,  and  geographical  barriers. 
Among  these  the  simplest  and  the  most  universal  is 
temperature.  We  will  therefore  speak  first  of  temperature 
regio7is.  And  among  organisms  the  simplest  in  their  re- 
lations are  plants.     Therefore  we  speak   first  of  all  of 

Botanical  Temperature  Regions. — As  we  travel 
from  equator  to  pole  we  pass  through  successive  zones 
of  temperature  ranging  from  80°  to  0°  F.  These  zones 
are  characterized  predominantly  by  different  groups  of 
plants.  We  have  first  a  region  of  palms  and  tree  ferns, 
corresponding  with  the  intertropical  zone;  then  a  region 
of  evergreen   hard-wood   trees  corresponding  with  the 


GEOGRAPHICAL  DISTRIBUTION   OF  ORGANISMS. 


473 


warm  temperate  zone;  then  a  region  of  deciduous  hard- 
wood trees,  corresponding  with  the  full  temperate  zone; 
then  a  region  of  conifers  and  birches,  corresponding  to 
the  cold  temperate  or  subarctic  zone;  then  a  treeless 
region,  corresponding  to  the  arctic  or  circumpolar  zone; 
and  finally  a  plantless  or  nearly  plantless  region,  occupied 
by  the  polar  ice  cap  (Fig.  332). 

Qualification. — i.  By  the  terms  region  of  palms, 
region  of  pines,  etc.,  we  mean  only  that  these  kinds  of 
trees  by  their  abundance  give  character  to  the  aspect  of 
field  and  forest. 

2.  We  have  drawn  lines  separating  these  regions,  but 
in  fact  they  shade  completely  into  one  another. 

3.  We  have  drawn 
the  separating  lines 
regular,  like  lines  of 
latitude,  but,  in  fact, 
they  are  irregular, 
like  isothermal  lines. 

Regions  in  Al- 
titude. —  These  re- 
gions being  tempera- 
ture regions,  similar 
regions  are  found  in 
ascending  mountains 
(Fig.  332).  A  moun- 
tain in  the  tropics  if 
it  reaches  perpetual 
snow  will  contain  all 
of  them,  while  moun- 
tains in  higher  lati- 
tude only  the  higher 
portions  of  the  se- 
ries. For  example,  in  the  Peruvian  Andes  we  have  all 
these    regions    successively  encircling    the   mountain — 


Fig.  332. — Diagram  showing  temperature 
zones  in  latitude  and  corresponding  zones 
in  altitude  :  i,  tropical ;  2,  temperate  ;  3, 
subarctic ;  4,  arctic  ;  5,  perpetual  snow, 
a,  b,  c,  mountams  in  tropical,  temperate, 
and  subarctic  regions  respectively. 


474   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

i.  e.,  a  region  of  palms  at  the  base,  a  region  of  hard-wood 
trees  higher  up,  then  a  region  of  pines,  a  treeless  region, 
and  a  plantless  region  or  perpetual  snow.  In  the  sierra, 
which  is  in  the  temperate  zone,  the  region  of  palms  is 
wanting,  but  all  the  others  are  present.  To  two  thou- 
sand feet  or  more  hard-wood  trees  predominate,  then 
pines  and  spruces  up  to  ten  thousand  to  twelve  thousand, 
then  shrubs  and  flowers,  and  finally  perpetual  snow. 

Zoological  Temperature  Regions.— Thus  far 
we  have  spoken  only  of  plants,  because  their  relations 
are  more  simple  and  easily  brought  out  on  account  of 
their  being  fixed  to  the  soil.  But  the  same  laws  govern 
animal  species  also.  Animals  also  have  their  tempera- 
ture regions.  For  example,  the  great  cats,  the  hyenas, 
the  great  pachyderms,  the  camels,  the  ostriches,  parrots, 
toucans,  the  reef-building  corals,  etc.,  are  tropical,  while 
the  polar  bear,  the  civets,  martens,  seals,  walruses,  and 
whales  are  predominantly  arctic. 

COMPLETER    DEFINITION    OF    TEMPERATURE    REGIONS. 

I.  The  range  of  any  organic  form  is  the  extent  or 
area  of  its  distribution.  It  is  most  restricted  for  spe- 
cies;  is  greater  for  genera,  because  when  a  species 
ceases  the  genus  may  be  continued  by  other  species  of 
the  same  genus.  It  is  still  greater  for  families,  because 
when  a  genus  ceases  the  family  may  still  continue  as 
other  genera  of  the  same  family,  etc.  The  range  is  ex- 
tensive in  proportion  to  the  largeness  of  the  taxonomic 
group.  For  example,  we  have  said  that  the  range  of 
the  conifers  in  the  sierra  was  from  two  thousand  to  ten 
thousand  feet.  This  is  the  range  of  the  order,  but  no 
species  extend  so  far.  If  we  take  the  genus  Pinus  we 
have  first  the  digger  pine  (P.  salnniana),  then  the  yellow 
pine   {P.  ponderosa),  then   the    sugar  pine   {P.  Lamberti- 


GEOGRAPHICAL   DISTRIBUTION   OF   ORGANISMS. 


475 


ana),  then  the  tamarack  pine  {J\  contorta),  then  the  /'. 
flex  Hi Sy  etc. 

2.  The  ranges  of  contiguous  species  shade  insensibly 
into  one  another  by  overlapping  interpenetration  and 
co-existence  on  the  margins. 

Each  species  is  most  abundant  and  vigorous  about 
the  middle  of  its  range,  and  becomes  less  and  less  numer- 
ous and  vigorous  on  the  margins  until  it  ceases  and 
gives  place   to  some  other  species.     Thus,  if  a  a'  (Fig. 


333)  be  the  range  of  species  A,  and^<^'  of  species -/5, 
then  the  rising  and  declining  curves  represent  the  rela- 
tive abundance  in  different  parts  of  their  ranges,  and 
a! b  their  overlap  or  area  of  coexistence  on  the  margins. 

3.  But  species  do  not  usually  grade  into  other  species, 
which  take  their  place,  in  specific  characters.  There  is 
not  usually  any  evidence  of  transmutation  of  one  spe- 
cies into  another.  One  species  seems  to  \i&  replaced  by, 
not  transmuted  into,  another  species.  The  change  is 
usually  by  substitution^  not  by  transmutation.  I  say  usu- 
ally because  sometimes  such  gradation  in  specific  char- 
acters is  found. 

Illustrations. — We  take  for  illustration  only  two  ex- 
amples:  (i)  The  sequoia,  or  big  tree,  and  redwood  exist 
to-day  only  in  California;  one  (the  redwood)  confined 
to  the  coast  range,  and  the  other  (the  big  tree)  to  the 
Sierra.  Now,  in  commencing  south,  wherever  found 
these  trees  are  perfect  in  all  their  specific  characters  of 
bark,  wood,  leaf,  and  fruit.  They  remain  substantially 
unchanged   throughout   their   range,    and   stop   at    their 


476   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

northern  limit  still  unmistakably  the  same.  They  do 
not,  and  apparently  can  not,  change  their  specific  char- 
acters. They  die  first.  (2)  Take  another  example,  the 
sweet  gum  (liquidambar)  of  the  eastern  coast.  This 
remarkably  distinct  form  ranges  from  Florida  to  the 
borders  of  the  Great  Lakes.  Throughout  all  this  wide 
range  it  is  precisely  the  same  unmistakable  species, 
characterized  by  its  peculiar  starred  leaf,  winged  twigs, 
spinous  burr,  and  fragrant  gum.  On  the  limits  of  its 
range  it  does  not  change  into  any  other  species,  but  sim- 
ply dies  out  and  is  replaced  by  others. 

These  are  illustrations  of  what  has  been  called  ^^per- 
manence of  specific  form."  It  is  as  if  species  originated,  no 
matter  how — say,  at  once  by  creation — in  their  present 
form,  somewhere  in  the  region  where  we  find  them,  and 
thence  spread  in  all  directions  as  far  as  physical  condi- 
tions and  struggle  for  life  with  other  species  would  allow, 
interlocking  there  with  other  contesting  species  and  co- 
existing with  them  on  the  common  border. 

4.  Barriers. — Faunas  and  floras  shade  gradually 
into  one  another  when  limited  only  by  temperature  con- 
ditions ;  but  if  there  should  be  an  impassable  physical 
barrier,  like  an  east  and  west  mountain  range,  or  a  des- 
ert or  sea,  then  the  fauna  and  flora  on  each  side  of  such 
barrier  will  differ  greatly  and  without  any  shading.  Thus 
the  species  north  and  south  of  the  Himalaya,  or  north 
and  south  of  the  Sahara,  differ  conspicuously  and  tren- 
chantly. It  is  again  as  z/ each  group  of  organisms  had 
originated  or  been  created  at  once  out  of  hand  just  as 
they  are  and  where  we  find  them,  and  spread  as  far  as 
they  could,  but  could  not  mingle  with  the  next  contigu- 
ous group  on  account  of  the  intervening  barrier. 

5.  North  and  South  of  the  Equator.— Again, 
there  are  temperature  zones  south  of  the  equator  as  well 
as  north  ;  but  none  of  the  species  found  in  temperate  or 


GEOGRAPHICAL  DISTRIBUTION   OF  ORGANISMS. 


477 


arctic  zones  north  are  the  same  as  those  found  in  similar 
zones  south.  It  is,  again,  as  //each  group  was  created 
as  it  is  and  where  we  find  it,  and  prevented  from  pass- 
ing to  similar  zone,  north  or  south,  by  the  torrid  tem- 
perature intervening. 


CONTINENTAL    FAUNAL    REGIONS. 

Thus  far  we  have  considered  onlv  temperature  con 
ditions;  now  we  take  up  other  limiting  conditions. 

If  continuous 
land  existed  all 
around  the  earth, 
then,  barring  des- 
ert regions,  there 
is  no  reason  to 
doubt  that  species 
would  range  all 
around,  and  there 
would  be  a  strict- 
ly zonal  arrange- 
ment of  species  de- 
termined by  tem- 
perature alone. 
But  the  continents 
are  widely  and  im- 
passably separat- 
ed by  oceans. 
Therefore  the  species  on  different  continents  are  wholly 
different.  It  is,  again,  as  if  each  continent  had  been 
populated  by  its  own  inhabitants,  suited  to  its  climate, 
just  as  we  find  it  now,  and  had  not  been  able  to  cross 
the  ocean  barrier  and  mingle. 

If  we  take  the  facts  in  detail  the  case  becomes  still 
stronger.  Let  Fig.  334  represent  a  north  polar  projec- 
tion  of  the  earth.     The   five  zones  are  represented  by 


Fig.  3-?4. — Polar  projection  of  the  earth  :  i, 
tropical ;  2,  temperate  ;  3,  subarctic  ;  4,  arc- 
tic ;  5,  polar  regions. 


478 


PHYSIOLOGY  AND    MORPHOLOGY    OF   ANIMALS. 


corresponding  numerals.  We  may  leave  out  No.  5,  as 
this  is  unknown  and  largely  uninhabited.  In  No.  4  the 
fauna  is  practically  the  same  all  around,  because  of  the 
close  approximation  of  the  lands  of  the  two  continents 
here  and  the  easy  communication  over  the  solid  ice. 

But  in  Nos.  3  and  2,  which  include  the  United  States 
and  Europe,  we  find  the  species  are  substantially  all  dif- 
ferent. Even  many  of  the  genera  and  some  families 
are  peculiar  to  each  continent.  Some  species,  indeed, 
are  representative  or  resembling  species,  but  not  identi- 
cal. To  this  general  statement  there  are  some  excep- 
tions, but  these  are  of  the  kind  which  prove  the  rule, 
or  rather  the  principle  on  which  the  rule  is  founded. 

Exceptions. — I.  Hardy  or  widely  migrating  species,  such 
as  geese  and  ducks.  These  are  the  same  on  the  two 
continents,  because  they  range  also  into  No.  4,  and  thence 
go  down  on  either  continent. 

2.  Introduced  species,  such  as  all  the  useful  plants  and 
domesticated  animals  and  all  the  noxious  weeds  and 
animal  pests — flies,  rats,  etc. — which  follow  the  footsteps 
of  civilization.  We  not  only  find  these  on  both  conti- 
nents, but  they  often  do  as  well,  or  even  better  in  their 
new  homes  than  in  their  native  places.  They  were  not 
in  those  new  homes  before  only  because  they  could  not 
get  there. 

3.  Alpine  Species. — It  is  a  curious  fact  that  animals 
and  plants  inhabiting  the  tops  of  high  mountains  in  Eu- 
rope and  in  the  United  States  are  extremely  similar,  and 
even  sometimes  identical,  even  though  so  widely  and 
impassably  separated.  The  explanation  of  this  will  come 
up  later. 

In  No.  I,  or  tropic  zone,  the  difference  is  still  greater; 
not  only  all  the  species  in  South  America  and  Africa  are 
wholly  different,  but  many  whole  families  are  entirely 
peculiar  to  one  or   the  other  continent.     For  example, 


GEOGRAPHICAL   DISTRIBUTION    OF   ORGANISMS.    470 

the  monkeys  are  found  in  both,  but  the  tailless  monkeys 
are  peculiar  to  the  Old  World,  while  the  prehensile 
tailed  monkeys  are  peculiar  to  America.  The  great 
pachyderms  are  peculiar  to  the  Old,  while  the  sloths  are 
peculiar  to  the  New.  Among  birds  the  great  family  of 
humming  birds,  containing  over  four  hundred  species, 
are  peculiar  to  America,  while  the  true  ostriches  are  con- 
fined to  Africa.  Among  plants,  the  great  family  of  cac- 
tuses, with  its  innumerable  species,  is  peculiar  to 
America. 

Nos.  2  and  3,  in  the  southern  hemisphere,  continue 
quite  distinct  in  South  America  and  Africa,  because  these 
are  still  widely  separated.  There  are  some  evidences, 
however,  derived  entirely  from  their  faunas  and  floras, 
that  they  were  once  more  appro.ximated  than  now. 

SUIU)lVISIO.\S    OF    CONTINENT.AL    F.\UNAS. 

We  have  already  spoken  of  subdivisions  of  these  de- 
termined by  temperature,  and  in  a  more  marked  manner 
by  east  and  west  barriers,  such  as  mountain  chains,  etc. 
But  they  are  divided  also  by  north  and  south  barriers. 
Thus  the  United  States  fauna  is  divided  in  a  very  marked 
way  by  north  and  south  mountain  chains  into  distinct 
faunal  regions.  By  the  Appalachian  range  the  division 
is  not  very  marked,  because  the  chain  is  not  very  high 
nor  very  long,  but  the  Rocky  Mountain  chain,  running 
the  length  of  the  continent,  and,  together  with  the  inter- 
mountain  deserts,  more  than  one  thousand  miles  wide, 
form  an  insuperable  barrier  to  most  species  of  animals 
and  plants.  So  that  California  is  a  very  distinct  faunal 
and  floral  region.  Similarly,  but  in  less  degree,  the 
Ural  separates  a  European  from  an  Asiatic  fauna  and 
flora. 

Special  Cases. — i.  Australia. — We  have  thus  far 
spoken  only  of  the  two  great  continental  masses,  east- 
32 


48o   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

ern  and  western,  but  there  is  another  land  mass  of  con- 
tinental proportions — viz.,  Australia.  This  we  take  as 
our  first  example  of  special  cases.  It  is  the  most  isolated 
of  continents,  and  the  most  distinct  of  all  faunal  regions. 
All  the  animals  and  plants  there  are  widely  different 
from  those  of  any  other  region.  To  show  the  greatness 
of  this  difference  we  take  up  more  particularly  one  class 
only — viz.,  the  mammals. 

Mammals  are  divided  into  three  subclasses :  Euthe- 
ria,  or  ordinary  typical  mammals,  found  everywhere  ex- 
cept Australia  ;  Metatheria,  or  marsupials ;  and  Proto- 
theria,  or  monotremes.  Eutheria  are  perfect  young-bear- 
ers ;  the  Prototheria  are  perfect  egg-layers ;  the  Meta- 
theria  are  intermediate.  The  young  of  these  are  born 
in  a  very  imperfectly  organized  condition,  and  embry- 
onic development  is  finished  in  the  marsupium  or 
pouch.  Now,  there  are  about  one  hundred  and  fifty 
species  of  mammals  in  Australia,  all  of  which  are  mar- 
supials and  monotremes.  Moreover,  monotremes  are 
found  nowhere  else,  and  of  marsupials  only  a  few  opos- 
sums are  found  elsewhere,  viz.,  in  America,  North  and 
South.  The  only  exception  to  this  sweeping  differ- 
ence is  the  existence  of  a  few  bats  and  a  few  rats 
— animals  especially  liable  to  dispersal  by  means  of 
floating  logs,  etc.,  and  therefore  to  accidental  intro- 
duction. 

2.  Madagascar. — This  very  large  island  off  the  east 
coast  of  Africa  is  separated  from  the  latter  by  the  wide 
and  deep  Mozambique  Channel.  Next  to  Australia,  it  is 
perhaps  the  most  distinct  of  all  faunal  regions.  It  may 
be  called  the  home  of  the  lemurs,  and  has  besides  many 
curious  forms.  All  the  mammals  are  peculiar,  except 
such  as  have  been  introduced.  But  it  is  well  to  remem- 
ber that  whatever  distant  resemblances  they  have  are 
mostly  with  those  of  South  Africa. 


GEOGRAPHICAL   DISTRIBUTION   OK  ORGANISMS. 


4<Si 


3.  Galapagos. — These  islands,  off  the  west  coast  of 
South  America,  strongly  attracted  the  attention  of  Dar- 
win during  his  voyage  on  the  Beagle  on  account  of  the 
singularity  of  their  fauna.  They  have  been  visited  by 
many  other  naturalists  for  the  same  reason.  There  are 
no  mammals  or  amphibians,  and  the  species  of  other 
groups  are  all  peculiar.  Among  them  are  found  many 
very  large  lizards  and  several  species  of  gigantic  land 
tortoises.  These  islands  are  separated  from  South  Amer- 
ica by  deep  water. 

4.  River  Mussels. — We  might  multiply  examples  in- 
definitely. We  take  one  more.  In  the  Altamaha  River, 
Georgia,  among  other  shells  found  there  is  one,  Unio 
spinosus,  with  needlelike  spines  an  inch  and  a  half  long. 
This  species  is  not  found  elsewhere  on  the  face  of  the 
earth.     How  did  it  get  there  ? 

In  all  these  cases  it  would  seem  as  if  the  groups  of 
animals  and  plants  had  been  made  and  put  there  where 
we  find  them,  and  are  not  found  elsewhere  because  they 
could  not  get  away. 

Marine  Faunas. — Thus  far  we  have  spoken  only 
of  land  organisms,  but  the  same  laws  are  true  in  less 
degree  for  marine  species. 

Temperature  Regions.— Here,  also,  of  course,  we 
have  temperature  regions,  and  consequent  gradations  by 
change  of  species  as  we  go  north  or  south.  Here,  also, 
at  certain  places  we  may  have  more  abrupt  changes. 
For  example,  on  the  east  coast  of  the  United  States  we 
have  two  abrupt  changes  of  coast  fauna,  one  at  Cape 
Cod  and  the  other  at  Cape  Hatteras.  Scarcely  a  single 
species  passes  from  north  to  south  of  these  points,  or 
vice  versa.  The  cause  is  found  in  the  marine  currents  off 
the  coast.  The  warm  Gulf  waters  coming  through  the 
straits  of  Florida  hug  the  coast  and  warm  the  shore 
waters   as   far   as   Cape  Hatteras  (<^  F'ig.  3.^6),  and  then 


482 


PHYSIOLaOY   AND    MORPHOLOGY   OF   ANIMALS. 


leave  the  coast.  The  icy-cold  waters  coming  out  of 
Baffin's  Bay  hug  the  New  England  coast  as  far  as  Cape 
Cod,  b,  giving  it  its  peculiarly  harsh  climate,  and  then 
disappear  and  become  a  deep  current.  Thus  the  sub- 
tropical fauna  is  carried  beyond  its  limits  to  Cape  Hat- 
teras,  while  the  arctic  shore  fauna  is  carried  beyond  its 
natural  limits  as  far  south  as  Cape  Cod,  making  three 
sharply  defined  shore  faunas. 

Continental  Shore  Faunas. — The  richest  marine 
faunas  are  along  the  shores  of  continents.  The  deep  sea 
is  an  impassable  barrier  to  these.  Therefore  the  faunas 
along  the  two  shores,  European  and  American,  of  the 
Atlantic  are  almost  wholly  different.  The  continent 
itself  is,  of  course,  a  still  more  effectual  barrier,  and 
therefore  our  Atlantic  shore  species  and  Pacific  shore 
species  are  wholly  different. 

Pelagic  Fauna. — Many  marine  species  swim  or 
fioat  freely  in  the  open  sea,  and  are  much  more  widely 
diffused,  being  limited  by  temperature  alone.  These 
are  called  pelagic  species. 

Abyssal  Fauna. — Again,  certain  species  live  only 
at  very  great  depths.     These  are  abyssal  species. 

Special  Cases. — There  are  special  cases  here  also 
— that  is.,  the  species  found  along  the  shores  of  iso- 
lated islands,  as  about  Australia,  Madagascar,  Gala- 
pagos, etc. 

Primary  Divisions  of  Land  Faunas.— Thus,  then, 
organic  forms  are  limited  in  range  in  every  direction  by 
many  kinds  of  physical  conditions.  They  are  limited 
north  and  south  by  temperature,  and  in  all  directions  by 
barriers  such  as  mountain  chains,  deserts,  and  oceans. 
Besides  these,  climate  and  soils  limit  especially  plants, 
and  these  limit  animals.  Thus  the  whole  earth  is  di- 
vided into  a  few  great  \)x'\vr^^xy  fan nal  regions,  and  these 
are    subdivided   into  prmnnces,   etc.     Many    schemes  of 


GEOGKAI'IIICAL   DISTRIBUTION   OF    ORGANISMS.    483 

primary  and  secondary  divisions  have  been  proposed. 
I  give  that  which  is  most  generally  adopted — viz.,  that  of 
Mr.  Sclater  and  Mr.  Wallace.  According  to  this  scheme 
there  are  six  primary  regions,  each  subdivided  into  four 
provinces.  The  primary  regions  are  :  i.  Palearctic,  in- 
cluding the  whole  of  the  Old  World  north  of  Sahara  and 
the  Himalayas.  2.  Ethiopian,  including  Africa  south  of 
Sahara.     3.  Oriental,  including  Asia  south  of  the  Hima- 


FiG.  335. — Map  of  the  world,  showing  the  six  primary  regions  of  Mr. 
Wallace. 


layas,  together  with  the  adjacent  islands,  Ceylon,  Java, 
Borneo,  Philippines,  etc.  4.  Australian,  including  Aus- 
tralia, New  Guinea,  New  Zealand,  and  Polynesia.  5. 
Neotropic,  including  South  America,  Central  America, 
and  the  Antilles.  6.  iVearctic,  including  all  North  Amer- 
ica north  of  Mexico.  These  regions  are  shown  on  map 
(Fig.  335)- 

These  are  each  subdivided  into  four  provinces,  as 
shown  in  the  following  sched4jle;  but  we  will  give  more 
particularly  only  those  of  our  own  Nearctic  region. 


484 


PHYSIOLOGY  AND    MORPHOLOGY   OF   ANIMALS. 


r  I.  European. 

1.  Palearctic J   2*  Mediterranean. 

•    3.   Siberian. 
[  4.  Manchurian. 
f  I.  Eastern. 

2.  Ethiopian j    -■  Western. 

I    3.  Soutliem. 
(  4.   Malagasian. 
r  I.  Indian. 

3.  Oriental |   2.  Ceylonese. 

'l  3.  Indo-Chinese. 
[  4.  Indo-Malayan. 
C  I.  Austro-Malayan. 

4.  Australian 1    2.  Australian 

1    3.  New  Zealandian. 
[  4.  Polynesian. 
C  I.  Chilian. 

5.  Neotropic J    2.  Brazilian. 

1    3.  Mexican. 

I 

[  4.  Antillean. 

[    I.  Californian. 

6.  Nearctic J    -■   Rocky  Mountain. 

I    3.  Alleghanian. 
[  4.  Canadian. 

The  subdivisions  of  the  Nearctic  are  given  in  Fig. 
336.  They  are  :  i.  Ca///<^r«/<T,  including  the  Pacific  bor- 
der from  Vancouver  Island  to  the  borders  of   Mexico. 

2.  Rocky  Mountains,  including  all  the  mountains  and 
desert  region,  and  extending  into  the  Mexican  plateau. 

3.  Alleghanian,  including  all  the  United  States  east  of  the 
plains  and  south  of  the  Great  Lakes.  4.  Canadian,  in- 
cluding all  north  of  i,  2,  and  3. 

Primary  Divisions  of  Marine  Faunas. — Sclater 
makes  of  these  also  six.  But  in  the  present  imperfect 
state  of  knowledge  the  simpler  classification  proposed 
by  Gill  seems  preferable.  Gill  divides  marine  faunas 
into  three  great  realms:  i.  North  Polar  or  Arctalian.  2. 
Tropicalian.     3.   South  Polar,  or  Notalian. 


GEOGRAPHICAL   DISTRIBUTION   OF  ORGANISMS.    485 

Theories  of  the  Origin  of  the  Distribution  of 
Organisms.— We  have  given  in  outline  the  tacts.  The 
question  is,  "  How  came  it  so  ?  "  Before  the  advent  of 
the  theory  of  evokition  the  most  rational  theory  was 
based  on  the  ideas  oi  permanence  of  specific  types  and  centers 
of  specific  origin,  as  if  each  species  was  made  in  its  pres- 
ent form  and  its  present  place,  and  thence  spread  as  far 


Fig.  336. 


as  it  could  without  changing  its  character.  It  did  not 
deny  some  variation,  but  always  within  certain  limits. 
If  the  centre  of  a  circle  represents  a  specific  type,  then 
radii  will  represent  the  variation  in  all  directions,  and 
the  circumference  the  limit  of  variations.  Some  species 
are  more  and  some  less  variable ;  the  circle  may  be 
larger  or  smaller,  but  in  all  there  is  a  limiting  line  be- 
yond which  it  is  impossible  to  go  without  destroying  the 


486    PHYSIOLOGY   AND    MORPHOLOGY    OF   ANIMALS. 

species.  Or,  again,  permanence  may  be  compared  to  a 
right  cylinder  standing  on  end.  It  may  lean  in  any 
direction  to  a  limit  and  right  itself,  but  pressed  too  far  it 
is  overthrown,  and  the  species  is  destroyed.  Within  these 
limits  species  are  made  according  to  a  plan  or  type  at 
once,  and  these  continued  by  generation  unchanged. 
They  are,  as  it  were,  struck  from  the  same  die,  until  the 
die  is  broken  or  worn  out  and  another  made.  Thus  the 
process  goes  on  by  an  alternation  of  supernatural  and 
natural  means.  The  origin  was  supernatural,  the  con- 
tinuance by  natural  process  of  generation.  The  mak- 
ing of  dies  was  supernatural,  the  coinage  was  natural. 

This  old  theory,  as  already  shown,  explains  many  of 
the  phenomena  given  above,  hut  not  all.  For  example  : 
I.  If  each  species  were  made  especially  for  a  certain 
place  and  environment,  then  it  ought  to  be  more  per- 
fectly adapted  to  that  place  than  any  other,  but,  on  the 
contrary,  introduced  species  of  ten  flourish  better  in  their  new 
than  in  their  old  homes.  2.  Again,  if  species  are  made  each 
in  its  own  place  and  spread  as  far  as  they  can  and  wher- 
ever they  can — as  indeed  they  do — then  the  amount  of 
difference  between  faunas  of  different  places  ought  to 
be  in  strict  proportion  to  the  impassableness  of  the  bar- 
riers between.  This  is  indeed  largely  true,  but  not  the 
whole  truth.  There  is  another  element  which  is  left  out — 
viz.,  the  element  of  time.  The  difference  is  proportioned 
to  the  impassableness  of  the  barrier  and  the  time  since  the 
barrer  was  set  up:  This  element  of  time  connects  the 
subject  with  the  idea  of  evolution  of  organic  forms 
throughout  all  geological  time.  In  a  word,  the  old 
theory  was  well  enough  for  the  present  condition  of 
things,  though  not  perfect  even  there,  but  fails  entirely 
to  connect  present  faunas  and  their  distribution  with 
those  of  previous  times.  The  study  of  the  present  alone 
is  but  a  flash-light  view  of  the  world,  and  therefore  the 


GEOGRArillCAl.   DISTRIBUTION   OF  ORGANISMS.    487 

world  seems  to  stand  still,  and  organic  forms  seem  to  be 
perma)ie?it.  Geology  alone  shows  us  the  world  in  con- 
tinuous motion,  in  continuous  change  by  evolution. 

In  the  ne7>.>  and  now  universally  adopted  view  there 
are  four  principles  to  be  borne  in  mind:  i.  The  origin 
of  organic  forms  "by  descent  with  modifications" — i.e., 
by  evolution — and  the  steady  forward  march  of  evolu- 
tion everywhere  and  through  all  time.  If  this  were  all, 
there  would  be  no  geographical  diversity  at  all.  2.  In 
different  places,  with  different  environments  and  isolated 
from  one  another,  evolution  took  different  directions,  and 
faunas  became  more  and  more  different  as  long  as  the 
isolation  continued.  If  this  had  been  all,  geographical 
diversity  would  by  this  time  have  been  far  more  extreme 
than  we  find  it  anywhere.  3.  But  from  time  to  time,  at 
long  intervals  in  geological  history,  there  occurred  wide- 
spread changes  in  physical  geography  and  climate  by 
which  barriers  were  removed  and  new  barriers  set  up. 
The  result  was  wide  migration  and  mingling  and  con- 
flict of  faunas,  and  consequently  more  rapid  evolution, 
but  at  the  same  time  a  decrease  of  geographical  diver- 
sity. 4.  A  re-isolation  in  new  localities,  and  a  commenc- 
ing process  of  divergence  which  is  still  going  on. 

Now,  the  last  of  these  periods  of  great  changes,  cli- 
matic and  geographic,  and  of  extensive  migrations  and 
minglings  and  conflict  of  faunas,  and  therefore  of  rapid 
evolution,  was  the  glacial  epoch,  or  ice  age.  It  is  evi- 
dent, then,  that  the  geographic  and  climatic  changes  of 
the  glacial  epoch  furnish  the  key  to  the  present  distri- 
bution of  species,  and,  conversely,  the  present  geo- 
graphic distribution  of  species  furnishes  a  key  to  the 
direction  of  migrations  of  that  time. 

A  full  discussion  of  this  interesting  subject  should 
be  preceded  by  a  course  in  geology,  but  an  outline  may 
be  brought  out  by  means  of  illustrative  examples. 


488    PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 

1.  Alpine  Species. — We  have  already  spoken  of  the 
remarkable  fact  that  alpine  species  of  plants  and  of  in- 
sects are  very  similar  (though  not  usually  identical)  in 
Europe  and  America,  although  so  widely  separated  and 
completely  isolated.  The  key  to  the  explanation  is 
found  in  the  additional  fact  that  they  are  both  similar 
to  arctic  species,  and  the  explanation  of  both  is  found  in 
the  migrations  of  the  glacial  epoch. 

We  have  already  seen  that  arctic  species  are  the 
same  on  the  two  continents,  because  they  are  circum- 
polar  and  in  substantial  connection  all  around.  Now, 
as  the  glacial  cold  came  on,  the  ice  sheet  advanced  south- 
ward, driving  the  arctic  species  before  it  on  both  conti- 
nents, until  they  invaded  and  occupied  all  America  to 
the  Gulf  and  all  Europe  to  the  Mediterranean.  As  the 
ice  sheet  retreated,  arctic  species  followed  it,  step  by  step, 
back  to  their  present  arctic  home.  But  many  individ- 
uals sought  arctic  conditions  by  retreating  up  the  slopes 
of  mountains,  and  were  left  stranded  there  on  both  con- 
tinents. Since  then  they  have  been  changed  somewhat 
in  different  directions  by  isolation,  but  the  time  has  not 
been  long  enough  for  great  divergence.  There  is  some 
difference,  but  not  so  great  as  the  wide  separation  would 
lead  us  to  expect. 

2.  Australia. — Of  all  known  faunas  and  floras,  this  is 
the  most  distinct.  It  is  so  because  longest  isolated.  We 
have  seen  that  all  its  mammals  are  nonplacentals — i.  e., 
marsupials  and  monotremes — except  a  few  introduced 
accidentally  or  by  man.  Furthermore,  that  monotremes 
are  found  nowhere  else,  and  marsupials  nowhere  else, 
except  a  few  opossums  in  America.  But  this  has  not 
always  been  so.  In  Jurassic  (middle  geological)  times 
the  earth  was  everywhere  abundantly  inhabited  by  mar- 
supials and  monotremes,  but  not  by  eutheres  or  typical 
mammals.     These  last  were  introduced  subsequently  in 


GEOGRAPHICAL   DISTRIBUTION   OF  ORGANISMS.    489 

the  Tertiary.  Therefore  we  conclude  that  Australia 
was  connected  with  other  continents  during  middle  geo- 
logical times,  and,  in  common  with  other  lands,  was 
inhabited  by  metatheres  and  prototheres,  but  that  before 
or  about  the  beginning  of  the  Tertiary  it  was  separated, 
and  has  remained  so  ever  since.  Therefore,  when,  by 
struggle,  migrations,  and  conflicts  on  the  great  theater 
of  evolution  Arctogcea  (Eurasia  and  North  America), 
eutheres  were  evolved  at  the  beginning  of  the  Tertiary, 
Australia  was  already  isolated,  and  they  never  got  there. 

3.  Africa. — The  fauna  of  Africa  south  of  Sahara  is 
very  distinct,  though  less  so  than  that  of  Australia.  The 
mammals  of  Africa  are  of  two  groups:  (i)  A  group  of 
large,  powerful  animals  somewhat  like  the  Eurasians,  but 
especially  like  the  Pliocene  Eurasians.  (2)  A  group  of 
small  animals  of  low  organization  (mostly  insectivores 
and  lemurs)  very  peculiar  to  Africa,  but  more  like  those 
of  Madagascar  than  any  other.  These  latter  we  shall 
call  indigenes  or  natives,  the  former  group  we  will  call 
invaders. 

Explanation. — In  preglacial  times  Africa  south  of 
Sahara  was  isolated,  and  inhabited  only  by  the  indigenes. 
Then  came  the  glacial  elevation,  opening  Africa  to  mi- 
grations from  the  north,  and  the  glacial  cold,  driving 
Pliocene  mammals  southward  into  Africa,  where  they 
were  shut  up  by  subsequent  changes  in  physical  geog- 
raphy. Then  came  the  struggle  between  the  invaders 
and  the  natives.  Both  were  sorely  changed,  the  in- 
vaders mainly  by  the  new  environment,  the  natives 
mainly  by  the  struggle  for  life.  The  final  result  was 
the  mixture  of  the  two  groups,  but  the  invaders  greatly 
predominated.  (See  Wallace's  Geographical  Distribu- 
tion of  Animals.) 

Island  Faunas. — We  have  spoken  thus  far  of  conti- 
nental faunas,  but   island   faunas  are  peculiarly  interest- 


^go  PHYSTOLOGV   AND    MORPHOLOGY    OF   ANIMALS. 

ing.  Islands  are  of  two  kinds — continental  islands  and 
oceanic  islands.  Continental  islands  are  outliers  of  con- 
tinents, separated  from  them  only  by  subsidence.  They 
have  a  geological  structure  similar  to  the  mother  conti- 
nent. Oceanic  islands  are  those  which  have  no  connec- 
tion with  any  continent,  but  have  been  built  up  from  the 
ocean  bed  by  volcanic  eruption  in  comparatively  recent 
geological  times.  The  fauna  of  continental  islands  is 
always  related  to  that  of  the  mother  continent,  differing 
from  it  in  proportion  to  the  width  of  ocean  separating 
and  the  ti/zie  of  separation.  The  fauna  of  oceanic  is- 
lands have  no  such  evident  relation  to  any  continent. 
As  examples  of  the  former  group  we  have  Ceylon,  Java, 
Borneo,  Sumatra,  Japan,  etc.,  as  appendages  of  Asia; 
Madagascar,  of  Africa  ;  the  British  Isles,  of  Europe  ;  the 
West  Indies,  of  North  America,  etc.  Of  oceanic  islands 
we  have  as  examples  the  Bermudas  and  Azores  in  the 
Atlantic,  and  all  the  Polynesian  Islands  in  the  Pacific. 

4.  Madagascar. — We  have  already  seen  that  the 
mammals  of  Madagascar  differ  greatly  from  those  of 
any  other  country,  but  that  their  nearest  alliance  is  with 
those  of  Africa.  We  now  add  that  this  alliance,  how- 
ever, is  only  with  those  we  called  the  indigenes,  and  not 
at  all  with  the  invaders. 

Explanatio7i. — In  preglacial  times,  when  Africa  was 
isolated  from  the  rest  of  the  world,  Madagascar  was 
connected  with  it,  and  both  were  inhabited  by  the  in- 
digenes. Before  the  glacial  epoch,  and  therefore  before 
the  invasion  of  Pliocene  mammals,  Madagascar  was  sep- 
arated from  the  continent  by  subsidence,  and  these  iso- 
lated indigenes  were  spared  the  invasion  and  conflict. 
Since  that  time  divergence  has  gone  on  until  now  the 
fauna  is  very  peculiar.  In  the  fauna  of  Madagascar  we 
probably  have  a  nearer  approach  to  the  original  inhab- 
itants of  both  than  we  now  have  in  the  African  indige- 


GEOGRAPIIICAL   DISTRIBUTION   OP'  ORGANISMS. 


491 


nous  group.  For  although  both  have  changed  with  time, 
yet  the  African,  more  than  the  Malagasian,  because  of 
the  struggle  which  the  former  alone  suffered. 

5.  British  Isles. — The  fauna  of  the  British  Isles  is 
substantially  the  same  as  that  of  the  European  conti- 
nent, but  there  are  some  very  significant  differences, 
(i)  It  is  i 2.x poorer  in  species,  and  this  is  especially  true 
of  Ireland.  For  example,  of  mammals — Europe  has 
ninety  species,  England  forty,  and  Ireland  only  twentv- 
two.  Of  reptiles  and  amphibians  Europe  has  twenty- 
two  species,  England  thirteen,  and  Ireland  four.  (2)  In 
many  cases  there  is  a  difference,  but  not  sufificient  to 
make  species;  the  differences  are  varietal  instead  of 
specific.     Such  are  the  facts. 

Explanation. — In  preglacial  times  the  British  Isles 
were  a  part  of  the  continent  and  inhabited  by  the  same 
animals.  In  glacial  times  they  were  covered  by  the  ice 
sheet  and  all  animals  were  destroyed  or  driven  south- 
ward. After  glacial  times  and  the  withdrawal  of  the 
ice  sheet  they  became  again  a  part  of  the  continent  and 
were  recolonized  from  Europe.  But  the  time  of  con- 
nection with  Europe  after  the  withdrawal  of  the  ice 
sheet  was  too  short  for  complete  colonization,  especially 
of  extreme  Ireland.  Some  species  had  not  colonized  at 
all  and  more  had  not  yet  reached  Ireland,  when  by  sub- 
sidence the  islands  were  cut  off  from  the  continent,  and 
at  the  same  time  Ireland  from  England.  This  entirely 
explains  the  comparative  poverty  of  the  fauna.  Now 
the  slight  differences.  The  time  has  been  too  short  and 
isolation  too  imperfect  for  great  divergence.  Diver- 
gence has  commenced,  but  has  gone  only  to  varietal  and 
not  to  specific  differences. 

6.  California  Coast  Islands. — Heretofore  I  have 
spoken  almost  wholly  of  faunas.  In  this  case  I  will 
deal  with  the  flora.      But  the  principles  are  the  same. 


492 


PHYSIOLOGY   AND    MORl'HOLOGY   OF   ANIMALS. 


The  large  islands  off  the  coast  of  southern  California 
have  all  the  characters  of  continental  islands.  They 
have  been  separated  from  California  in  comparatively 
recent  times.  The  flora  of  these  islands  is  very  peculiar. 
Out  of  three  hundred  species  described,  at  least  fifty 
are  found  nowhere  else.  The  others  are  similar  to 
those  in  California.  But  in  the  California  flora  it  is 
necessary  to  distinguish  two  groups — viz.,  a  distinctively 
Californian  group  and  a  group  which  is  more  widely 
diffused.  The  former  I  shall  call  indigenes  or  natives; 
the  latter  are  probably  invaders.  Now,  it  is  the  distinc- 
tively Californian  group  only  that  is  found  on  the  islands. 
These  are  the  facts.     Now  the 

Explanation. — Before  the  glacial  epoch  the  islands 
were  a  part  of  the  mainland  and  all  had  a  common  flora 
— viz.,  the  group  I  have  called  indigenes.  Then  came 
the  separation  by  subsidence  and  the  isolation  of  many 
indigenes  on  the  islands.  Then  came  the  glacial  cold 
and  the  invasion  of  California  by  a  northern  flora,  the 
struggle  between  invaders  and  natives,  the  destruction 
of  some  native  species,  and  the  modification  of  both  na- 
tives and  invaders  by  new  environment  and  by  conflict. 
The  final  result  was  the  California  flora  of  to-day.  The 
island  flora  was  spared  the  invasion  and  the  conflict. 
It  has  been  changed  less  than  the  native  Californian. 
We  have  in  them  a  nearer  approach  to  the  preglacial 
flora  of  both. 

7.  Oceanic  Islands  and  their  Fauna. — Oceanic  islands 
are  built  up  from  the  sea  bottom  mostly  by  volcanic 
action,  and  have  never  had  any  connection  with  a  conti- 
nent. We  see  these  forming  now.  When  first  formed 
they  are,  of  course,  uninhabited.  They  receive  their 
species,  animals  and  plants,  as  gifts  from  the  sea — mere 
floating  waifs  brought  by  waves  and  currents.  Their 
fauna  and   flora  are   always   peculiar  because   isolated, 


GEOGRAPHICAL   DISTRIBUTION  OF  ORGANISMS.    403 

but  with  affinities  connecting  with  several  different  lands. 
The  predominance  of  affinities  will  depend  partly  on 
proximity  and  partly  on  the  direction  of  winds  and  cur- 
rents. Mammals  (except  bats)  and  amphibians  are  en- 
tirely wanting  (unless  introduced  by  man)  because  these 
are  not  apt  to  be  drifted  on  logs,  as  are  some  reptiles. 


INDEX 


Abyssal  fauna,  482. 

Anabolism,  286. 

Analogy  versus  homology,  241- 
24S. 

Anatomy,  definition  of,  8  ;  phil- 
osophical, 241-281. 

Animal  functions,  classification 
of,  23-25. 

Animal  life,  organs  and  func- 
tions of,  26-2S. 

Animals  and  plants,  distinction 
between,  4. 

Animals,  general  structure  of, 
11-23. 

Animal  structure,  general  laws 
of,  241-281. 

Aortic  arches,  origin  of,  390. 

Arteries,  3S2. 

Astigmatism,  115. 

Binocular  perspective,  149. 

Blind  spot,  132. 

Blood,  globules  of,  347-349,  351, 
352,  353  ;  plates  of,  349  ;  chem- 
ical composition  of,  349 ; 
plasma,  350  ;  coagulation  of, 
350 ;  functions  of,  351  ;  origin 
and  history  of,  352  ;  compar- 
ative morphology  of,  354  ;  em- 
bryonic development  of,  356  ; 
33 


circulation  of,  in  man,  375- 
380  ;  in  reptiles  and  amphibi- 
ans, 385  ;  in  arthropods,  400  ; 
in  bivalves,  402  ;  in  gastro- 
pods, 403  ;  in  cephalopods, 
404  ;  in  echinoderms,  405  ;  in 
insects,  408. 

Blood  system  of  mammals,  385  ; 
of  birds,  385  ;  of  reptiles  and 
amphibians,  385  ;  of  fishes, 
387  ;  of  arthropods,  399 ; 
of  bivalves,  401  ;  of  gastro- 
pods, 403 :  of  cephalopods, 
404  ;  of  echinoderms,  404  ;  of 
coelenterates,  406  ;  of  proto- 
zoans, 408  ;  of  insects,  408. 

Blood  vessels,  382. 

Botanical  temperature  regions, 
472. 

Brachyopy,  113. 

Brain  of  man,  30,  72  ;  convolu- 
tions of,  34 ;  interior  structure 
of,  35  ;  microscopic  structure 
of,  35  ;  embryonic  develop- 
ment of,  37  ;  functions  of 
parts  of,  39-45- 

Brain  of  vertebrates,  72. 

Brain,  relative  size  of,  73,  74. 

Breathing,  mechanics  of,  361, 
369  ;  transition  from  gill  to 
495 


496   PHYSIOLOGY   AND    MORPHOLOGY   OF   ANIMALS. 


lung,  373  ;  of  arthropods,  399  ; 
of  bivalves,  401  ;  of  gastro- 
pods, 403  ;  of  echinoderms, 
404  ;  of  insects,  411. 

Capillaries,  3S3. 

Cephalization,  82. 

Cerebellum,  32,  40. 

Cerebro-spinal  system,  29. 

Cerebrum,  31,  39;  relative  size 
of,  76.  - 

Chromatism,  107. 

Circulation  of  blood  in  man, 
375-380;  in  reptiles  and  am- 
phibians, 385  ;  in  arthropods, 
400  ;  in  bivalves,  402  ;  in  gas- 
tropods, 403  ;  in  cephalopods, 
404;  in  echinoderms,  405  ;  in 
insects,  40S. 

Classification,  definition  of,  9. 

Classification  of  animals,  out- 
line of,  70,  71. 

Classification  of  fishes  by  scales, 
460. 

Claws,  454. 

Coats  of  the  eyeball,  loi. 

Color-blindness,  139-141. 

Color  perception,  137. 

Colors,  primary,  136. 

Contact,  kinds  of,  97. 

Continental  faunal  regions, 
477-4S1. 

Contents  of  the  eye,  103. 

Corals,  466-469. 

Corpus  striatum,  41. 

Cranial  nerves.  46,  54. 

Dental  formulae,  299. 
Diabetes,  cause  of,  449. 
Digestion,      mouth,      292-310  ; 

stomach,  310-318  ;  intestinal, 

319-331- 


Digestive  system  in  inverte- 
brates, 331-346. 

Dim-sightedness,  115. 

Distribution  of  organisms,  the- 
ories of  the  origin  of,  485- 
489. 

Ear,  human,  structure  of  the, 
174-181  ;  mode  of  action  of 
the  whole,  181  ;  functions  of 
parts  of,  181,  1S2  ;  of  birds, 
1S3  ;  of  reptiles,  183  ;  of  fishes, 
184  ;  of  mollusks,  185  ;  evo- 
lution of  the,  187,  188. 

Elbow  joint,  252. 

Embryology,  definition  of,  9. 

Emmetropy,  113. 

Eye  of  mammals,  156  ;  of  birds, 
157  ;  of  reptiles,  i|i8  ;  of  fishes, 
159  ;  of  cephalopods,  163  ;  of 
arthropods,  164. 

Eye  of  man,  its  shape,  setting, 
etc.,  99;  muscles  of,  100; 
coats  of  the  ball,  loi  ;  linings, 
103  ;  formation  of  image  in, 
103  ;  necessity  of  lenses  in, 
104;  application  of  principles 
to,  105  ;  compared  with  cam- 
era, 10.7  ;  chromatism  in,  107  ; 
aberration  in,  108 ;  adjust- 
ment of,  for  distance,  108  ;  ad- 
justment of,  for  light.  III  ; 
structure  of  the  iris  in,  112; 
normal  sight  of,  113;  defects 
in  sight  of,  113-115  ;  retina 
of,  116-122. 

Fauna  and  flora,  471. 

Feathers,  structure  of,  457 ; 
gradation  of,  to  hairs,  458. 

Fishes,  classification  of,  by  re- 
spiratory organs,  372,  373. 


INDEX. 


497 


Food,  definition  of,  288  ;  kinds 
of,  288  ;  milk  as,  289  ;  uses 
of,  289 ;  distinctive  uses  of 
the  kinds  of,  290  ;  preparation 
of,  291  ;  saccharization  of, 
310;  peptonization  of,  313; 
chylification  of,  323  ;  emulsi- 
fication  of,  324. 

Forearm,  252. 

Fore  limbs,  various  figures  of, 

251-253- 
Fovea,  130. 
Frontal  lobe,  relative  size  of,  82. 

Ganglia,  functions,  of,  86. 
Ganglionic  system,  67-70. 
General  principles,  1-28. 
Glottis  and  its  vocal  cords,  207. 
Glycogeny   and    its   relation  to 
vital  force  and  vital  heat,  443- 

449- 
Gray   matter,    relative   amount 

of,  74.  75- 

Hair,  453. 

Hand  and  foot  bones,  253. 

Hand,  modifications  of,  254. 

Harmonic  relations,  illustra- 
tions of,  472. 

Heart,  structure  of  the,  380.  390. 

Heel,  position  of  the,  256. 

Hind  limbs,  255. 

Hip  girdle,  256. 

Histology,  definition  of,  8. 

Homology  of  vertebrates,  248- 
266. 

Hoofs,  455. 

Horns,  455. 

Horopter,  146. 

Hyperopy,  114. 

Island  faunas,  489-493. 


Joints,  224. 

Katabolism,  2S6,  415-451. 

Kidneys,  place  and  form  of,  425  ; 
excretory  ducts  of,  426  ;  pelvis 
of,  427  ;  minute  structure  of, 
427  ;  function  of,  429 ;  com- 
pared with  lungs,  430;  in 
mammals,  birds,  reptiles,  and 
amphibians,  433  ;  in  insects 
and  mollusks,  434. 

Knee,  position  of  the,  256. 

Land  faunas,  primary  divisions 
of,  482-484. 

Larynx,  204  ;  structure  of,  206  ; 
muscles  of,  209  ;  as  a  musical 
instrument,  210. 

Law  of  peripheral  reference,  64. 

Limb  motion,  227-230. 

Limbs,  signification  of,  264 ; 
origin  of,  264. 

Linings  of  the  eye,  102. 

Liver,  position  and  structure  of, 
440-442  ;  function  of,  442. 

Locomotion,  230-232. 

Lungs  of  man,  358  ;  structure  of, 
359  ;  application  of,  to  breath- 
ing, 362 ;  of  birds,  366 ;  of 
amphibians,  367 ;  compared 
with  kidneys,  430-433. 

Lymphatic  glands,  function  of, 
414. 

Lymphatic  system,  411-414. 

Manus  and  pes,  257. 
Marine  faunas,  482. 
Medulla,  32,  40. 
Membranes,  30. 
Mollusca,  structure  of,  277. 
Morphology,    definition    of,    8 ; 
general  laws  of,  241-281. 


498 


PHYSIOLOGY    AND    MORPHOLOGY   OF   ANIMALS. 


Motion  and  locomotion,  227-232. 

Muscle  and  skeleton,  compara- 
tive morphology  and  physi- 
ology of,  232-240. 

Muscles  of  the  eye,  100. 

Muscle,  voluntary,  221  ;  invol- 
untary, 223. 

Muscular  system,  219-223. 

Myopy,  113. 

Nails,  454. 

Nearsightedness,  113. 

Nerve  force  versus  electricity, 
64. 

Nerves,  49-65  ;  structure  of,  56  ; 
function  of,  56. 

Nervous  system  of  man,  29. 

Nervous  system  of  vertebrates, 
70-83 ;  of  invertebrates,  84- 
93  ;  of  arthropods  and  anne- 
lids, 84;  of  mollusks,  89;  of 
radiates,    92  ;    of   protozoans, 

93- 
Number  of  bones  in  man,  224. 
Nutritive  functions,  283-287. 

Old-sightedness,  114. 

Optic  lobes,  32,  41. 

Organs,  rudimentary  and  use- 
less, 258. 

Oversightedness,  114. 

Owen's  classification  of  mam- 
mals, 77. 

Pelagic  fauna,  482. 
Perspective,  forms  of,  151. 
Physiology,  definition  of,  8. 
Plexuses,  69. 
Pons  Varolii,  32. 
Presbyopy,  114. 
Protozoa,  structure  of,  279. 


Radiata,  structure  of,  278. 

Relation  of  plants  to  animals  in 
regard  to  creation  of  animal 
force,  424. 

Respiration,  costal,  363  ;  dia- 
phragmatic or  abdominal,  364 ; 
of   mammals,  366 ;    of   birds, 

366  ;  of  reptiles,  366  ;  of  the 
tortoise,  367  ;  of  amphibians, 

367  ;  of  fishes,  368  ;  function 
of,  in  animal  economy,  417  ; 
chemistry  of,  41S  ;  purpose  of 
combustion  in,  419. 

Respiratory    organs,    358,    399, 

403,  404,  409. 
Retina   and   its  functions,   116- 

122. 

Saliva,  composition  and  use  of, 
294. 

Salivary  glands,  292  ;  structure 
of,  293  ;  excitation  of,  294. 

Scales,  460;  of  reptiles,  461. 

Secretion  versus  excretion,  416. 

Serial  homology  of  vertebrates, 
261-266 ;  of  arthropods  and 
annelids,  267-277. 

Shells,  turtle,  462  ;  mammalian, 
463  ;  of  insects,  463  ;  of  crusta- 
ceans, 464  ;  of  mollusks,  464- 
466  ;  of  echinoderms,  466. 

Shoulder  girdle,  250. 

Shoulder  joint  and  fore  limb, 
227. 

Sight,  sense  of,  98. 

Size  and  distance,  judgments  of, 

153- 

Skeletal  system,  223-232. 

Skin,  function  of,  435  ;  struc- 
ture of,  436-438  ;  comparative 
morphology    and    physiology 


INDEX. 


499 


of,  438-440 ;  of  vertebrates, 
452  ;  importance  of,  in  classifi- 
cation, 453. 

Smell,  sense  of,  and  its  organ, 
188-193. 

Special  sense,  relation  of,  to  gen- 
eral sensibility,  94-96. 

Spinal  column,  226. 

Spinal  cord,  45-49. 

Spinal  nerves,  46,  54. 

Spinal  or  reflex  system,  function 
of,  66. 

Sponges,  skeletal  deposits  in, 
469. 

Structure  of  vertebrates,  general 
plan  of,  249. 

Sudorific  glands,  437 

Taste,  sense  of,  and  its  organ, 
193-198. 

Taxonomy,  definition  of,  9. 

Teeth  in  vertebrates,  295-310 ; 
mammalian,  296;  composition 
of,  297  ;  kinds  of,  297  ;  varia- 
tion of,  298  ;  relative  size  of, 
298  ;  number  of,  and  relative 
number  of  kinds  of,  298  ;  mo- 
lar, structure  of,  300  ;  origin 
of  structure  of  herbivorous, 
302  ;  of  whales,  304  ;  of  birds, 
305  ;  of  reptiles,  305  ;  origin 
of  mammalian,  30S  ;  of  fishes, 
308. 

Temperature  regions,  definition 
of,  474-477- 


Thalamus,  33,  41. 

Theca,  468. 

Three  kingdoms  of  Nature,  rela- 
tion of  the,  1-8. 

Touch,  sense  of,  and  its  organs, 
19S-204. 

Ungulates,  classification  of,  by 

foot  structure,  257. 
Urine,  composition  of,  429,  430. 

Veins,  382. 

Vibrations,  perception  of,  g6. 

Vision,  122-155  ;  first  law  of, 
122  ;  second  law  of,  126  ;  third 
law  of,  144;  two  fundamental 
laws  of,  149;  erect,  129;  bin- 
ocular, 142  ;  double,  142  ;  sin- 
gle, 143  ;  limitation  of,  151. 

Visual  purple,  121. 

Voice,  204-218;  simple,  204; 
singing,  210;  speaking,  211; 
comparative     physiology    of, 

213  ;  in  birds,  214  ;  in  reptiles, 

214  ;  in  fishes,  214  ;  in  arthro- 
pods, 216. 

Waste  tissue,  290,  420. 
Whales,     mouth    armature    of, 
304  ;  mode  of  feeding  of,  305. 

Wrist  joint,  253. 

Zoological  temperature  regions, 

474. 
Zoology,  definition  of,  8-10  ;  de- 
partments of,  10. 


THE    END, 


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clear  and  eiiergeiic." — Lhurchman. 

TT I  STORY  OF    THE   CONFLICT  BETWEEN 
-Ll      J^ELIGION   AND    SCIENCE.     By  Dr.   John   William 
Draper.     i2mo.     Cloth,  $1.75. 

"The  key-note  to  this  volu-ne  is  found  in  ihe  antagonism  between  the  prrgresfivo 
tendencies  of  the  hmrian  mind  and  the  preie' sionsof  eccUsi.iftical  authoriiy,  «s  de- 
veloped in  the  hi^toiy  of  ni'idern  ^cience.  No  pe\i,ais  wriier  has 'real  ed  ihesiiiject 
fro:n  this  point  of  view,  and  the  present  monogiapli  will  be  f(  und  to  posse^s  no  less 
originality  of  conception  than  vigor  of  reasonini'.  and  wealth  of  eiudition." — New  V^rk 
Tribune. 

/I  CRITICAL  HISTORY  OF  FREE  IH  OUGHT 
■^     IN  REFERENCE   TO   THE  CHRISTIAN  RELIGION. 

By  Rev.  Canon  Adam  Storey  Farrar,  D.  D.,  F.  R.  S.,  etc. 

i2mo.  Cloth,  $2.00. 
"  A  conflict  mi^ht  naturally  be  anticipated  between  the  r'-a^oninE  faculties  of  man 
and  a  religion  which  c'aims  the  right,  on  superhuman  auihorty,  to  impose  limits  on 
the  fi'jld  oT  manner  of  their  exercise.  It  is  the  thief  of  the  rnovements  ol  t'lee  thought 
which  it  is  my  pirpose  to  d::scnbe,  in  their  historic  succession,  and  their  connection 
with  intellectual  causes.  We  must  ascertain  the  tacts,  discover  the  causes,  and  lead 
the  moral." — The  Author. 

/CREATION  OR    EVOLUTION 7     A  Philosophical 
^-^     Inquiry.    By  George  Ticknor  Curtis.    121110.     Cloth,  $2.00. 

"  k  treatise  on  the  great  questi  'U  of  Creation  or  Evolution  bv  one  who  is  neither  a 
naturalist  nor  theologisn,  and  wno  does  not  pr  ifess  to  brin?  to  the  discussion  a  special 
equipment  in  either  of  the  sciences  which  the  controversy  ir'ays  au^ui'^t  ench  other, 
may  seem  strange  at  first  si sht;  but  Mr  Curtis  wi'l  satisfy  the  reader,  be'bre  many  pages 
have  been  tnrneil.  thai  he  has  a  suiistantial  contribution  to  make  to  the  debaie,  and  that 
his  boTk  is  one  to  be  treated  with  respect  Hi?  part  is  to  a"plv  to  'he  reasonintis  of  the 
men  of  'Cieiice  the  risid  scrutiny  with  which  the  lavv  yei  is  accustomed  to  test  the  value 
and  pertinency  of  testimony,  and  the  legitimacy  of  inferences  from  established  facts." 
— New  Ynrk  Tribune. 

"  Mr.  Curtis's  bo'ik  is  honorably  distinguished  from  '  sadly  too  great  proportion  of 
treatises  which  profess  to  discuss  the  re'ation  of  scientific  theories  to  religion,  by  its 
author's  thorough  acquaintance  with  I  is  subject,  his  scrupulous  fairness,  and  remark- 
able freedom  from  passion." — London  Literary  World. 


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^lONEERS  OF  EVOLUTION,  from  Thales  to 
htixley.  By  Edward  Clodd,  President  of  the  Folk-Lore 
Society  ;  Author  of  "  The  Story  of  Creation,"  "The  Story  of 
'Primitive'  Man,"  etc.     With  Portraits.     i2mo.     Cloth,  |i. 50 

••The  mass  of  interesting  material  which  Mr.  Clodd  has  got  together  and 
woven  into  a  symmetrical  story  of  the  progress  from  ignorance  and  theory  to 
knowledge  and  the  inteligent  recoiding  of  fact  is  prodigious.  .  .  .  The 
'goal '  to  which  Mr.  Clodd  leads  us  in  so  masterly  a  fathion  is  but  the  start- 
ing point  of  frish  achievements,  and,  in  due  course,  fresh  theories.  His 
book  furnishes  an  important  contribution  to  a  literal  education.'— Z.^«(fc?« 
Daily  Chronicle. 

"  We  are  always  glad  to  meet  Jlr.  Clodd.  He  is  never  dull ;  he  is  always 
well  infor.-ned,  and  he  says  \\hat  he  has  to  say  with  clearness  and  precision. 
.  .  .  The  interest  intensifies  as  Mr.  Clodd  attempts  to  show  the  part  really 
played  in  the  growth  of  the  doctrine  of  evolution  by  men  like  Wallace,  Par- 
win,  Huxley,  and  Spencer.  .  .  .  We  commend  the  book  to  those  who  want 
to  know  what  evolution  really  means. "—Z,<?«</£>«  Times. 

"  This  is  a  book  which  was  needed.  .  .  .  Altogether,  the  book  could 
hardly  be  better  dene.  It  is  luminous,  lucid,  orderly,  and  temperate.  Above 
all,  it  is  entirely  free  from  personal  partisanship.  Each  chief  actor  is  sym- 
pathetically treated,  and  friendship  is  seldom  or  never  allowed  to  overweight 
sound  judgment." — London  Academy. 

"  We  can  assure  the  reader  that  he  will  find  in  this  work  a  verj'  u?eful  guide 
to  the  lives  and  labors  of  leading  evolutionists  of  the  past  and  pretent. 
Especially  serviceable  is  the  account  of  Mr.  Herbert  Spencer  and  his  share  in 
rediscovering  evolution,  and  illustrating  its  relations  to  the  whole  fie!d  of 
human  knowledge.  His  forcible  style  and  wealth  of  metaphor  make  all  that 
Mr.  Clcdd  writes  arrestive  and  intere-ting." — London  Literary  World. 

"  Can  not  but  prove  welcome  to  fair-minded  men.  .  .  .  To  read  it  is  to 
have  an  object-lesson  in  the  meaning  of  evolution.  .  .  .  There  is  no  bettei 
book  on  the  subject  for  the  general  reader.  .  .  .  No  one  could  go  through 
the  book  without  being  both  refreshed  and  newly  instructed  by  its  masterly 
survey  of  the  growth  of  the  most  powerful  idea  of  modem  times." — Tht 
Scotsman. 


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MODERN    SCIENCE    SERIES. 

Edited  by  Sir  John  Lubbock,  Bart.,  F.  R.  S. 

'J^HE   CA  USE  OF  AN  ICE  AGE.     By  Sir  Robert 
*         BALt,  LL.  D.,  F.  R.  S.,  Royal  Astronomer  of  Ireland  ;  author 
of  "  Siar  Land,"  "  The  Story  of  the  Sun,"  etc. 

"  Sir  Robert  Ball's  book  is,  as  a  matter  of  course,  admirably  written.  Though  but  a 
mall  on;,  it  is  a  mo?t  impuitant  contribution  to  ^^uXo^y ."  —  London  Saturday  Review. 

"  A  fascinating  subject,  cleverly  related  and  almost  colloquially  discussed  " — Phila- 
ielphia  t-'ublic  Ledge)'. 

^HE  HORSE:    A   Study  in    Natural    History.      By 
■*        William  H.  Flower,  C.  B.,   Director  in  the  British   Natural 
History  Museum.     With  27  Illustrations. 

"  The  author  admits  that  there  are  3,800  separate  treatises  on  the  horse  already  pub 
lished.  but  he  thinks  tt.«£  he  can  add  something  to  the  amount  of  useful  information 
now  before  the  public,  and  that  something  not  heretofore  written  will  be  found  in  this 
book.  The  volume  gives  a  large  amount  of  information,  both  scientific  and  practical, 
on  the  noble  animal  of  which  it  treats." — Neiv  York  Cemmercial  Advertiser. 

^HE  OAK :   A  Study  in   Botany.     By  H.  Marshall 

*■        Ward,  F.  R.  S.     With  53  Illustrations. 
"  From  the  acorn  to  the  timber  which   has  figured  so  gloriously  in  English  ships 
and  houses,  the  tree  is  fully  described,  and   all  its  living  and  preserved  beauties  and 
virtues,  in  nature  and  in  construction,  are  recounted  and  pictured." — Brooklyn  Eagle. 

TH NO  LOGY  IN  FOLKLORE.      By  George  L. 
GoMME,  F.  S.  A.,  President  of  the  Folklore  Society,  etc. 
"The  author  puts  forward  no  extravagant  assumptions,  and  the  method  he  points 
out  for  the  comparative  study  of  folklore  seems  to  promise  a  considerable  extension  of 
knowledge  as  to  prehistoric  times  " — Independent. 

^HE    LAWS   AND    FROFERTLES    OF   MAT- 

-*         TER.      By  R.  T.  Glazebrook,   F.  R.  S.,  Fellow   of  Trinity 

College,  Cambridge. 

"  It  is  astonishing  how  interesting  such  a  book  can  be  made  when  the  author  has  a 

perfect  mastery  of  his  subject,  as   Mr.  Glazebrook  has.      One  knows  nothing  of  the 

world  in  which  he  lives  until  he  has  obtained  some  insight  of  the  properties  of  matter 

as  e.xplained  in  this  excellent  work." — Chicago  Herald. 

'J^HE  FA  UNA  OF  THE  DEEP  SEA.    By  Sydney 

■•        J.   HiCKSON,  M.  A.,  Fellow  of  Downing   College,  Cambridge 

With  23  Illustrations. 

"That  realm  of  mystery  and  wonders  at  the  bottom  of  the  great  waters  is  gradually 

being  mapped  and  explored  and  studied  until  its  secrets  seem  no  longT  secrets.   .   .   . 

This  excellent  book  has  a  score  of  illustrations  and  a  careful  index  to  add  to  its.  value, 

ind  in  every  way  is  to  be  commended  for  its  interest  and  its  scientific  merit." — Chicag, 

TiHtes. 

Each,   i2mo,  cloth,  $1.00. 

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jr\EGENERATION.      By  Professor   Max   Nordau. 
-^-^    Translated  from  the  second  edition  of  the  German  work.     8vo. 
Cloth,  $3.50. 

"  A  powerful,  trenchant,  savage  attack  on  all  the  leading  literary  and  artistic  idoir 
of  the  time  by  a  man  of  great  intellectual  power,  immense  range  of  knowledge,  and 
the  possessor  of  a  lucid  style  rare  among  German  writers,  and  becoming  rarer  every- 
where, owing  to  the  very  influences  which  Nordau  atiacks  with  such  unsparing  energy, 
such  eager  hatred."— i,o«rt'o«  Clirotiicle. 

"  The  wit  and  learning,  the  literary  skill  and  the  scientific  method,  the  righteous 
indignation,  and  the  ungoverned  prejudice  displayed  in  Herr  Max  Nordau's  treatise  on 
'  Degeneration,'  attracted  to  it,  on  its  first  appearance  in  Germany,  an  attention  that 
was  partly  admiring  and  partly  astonished." — Loudon  Siandaid. 

"  Let  us  say  at  once  that  the  English-reading  public  should  be  grateful  for  an 
English  rendering  of  .Max  Nordau's  polemic.  It  will  provide  society  with  a  subject 
that  may  last  as  long  as  the  present  Government.  .  .  .  We  read  the  pages  without 
finding  one  dull,  sometimes  in  reluctant  agreement,  sometimes  with  amused  content, 
sometimes  with  angry  indignation." — London  Saturday  Review. 

"  Herr  Nordau's  book  fills  a  void,  not  merely  in  the  systems  of  Lombroso,  as  he 
says,  but  in  all  existing  systems  of  Knglish  and  American  criticism  with  which  we  are 
acquainted.  It  is  not  literary  criticism,  puie  and  simple,  though  it  is  not  lacking  in 
literary  qualities  of  a  high  order,  but  it  is  something  which  has  long  been  needed.  .  .  . 
A  great  book,  which  every  thoughtful  lover  of  art  and  literature  and  every  serious 
student  of  sociology  and  morality  should  read  carefully  and  ponder  slowly  and  wisely." 
— Richard  Henry  Stoddard,  in  the  Mail  and  Express. 

"'ihe  book  is  one  of  more  than  ordinary-  interest  Nothing  just  like  it  has  ever 
been  written.  Agree  or  disagree  with  its  conclusions,  wholly  or  in  p.irt,  no  one  can  fail 
to  recoijnize  the  force  of  its  argument  and  the  timeliness  of  its  injunctions," — Chicago 
Eziening  fast. 


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EN  I  US  AND  DEGENERATION.  A  Study  in 
Psychology.  By  Dr.  William  Hirsch.  Translated  from  the 
second  edition  of  the  German  work.  Uniform  with  "  Degen- 
eration."    Large  8vo.     Cloth,  $3.50. 

"The  first  intelligent,  rational,  and  scientific  study  of  a  great  subject  ...  In  the 
development  of  his  argument  Dr.  Hirsch  frequently  finds  it  necessary  to  attack  the 
positii>ns  assumed  by  Nordau  and  Lombroso,  his  two  leading  adversaries.  .  .  .  Only 
calm  and  sober  reason  endure.  Dr.  Hirsch  possesses  that  calmnefs  and  sobriety.  His 
work  will  find  a  permanent  place  among  the  authorities  of  science." — N.  \  .  Herald. 

"Dr.  Hirsch's  researches  .ire  intended  to  biing  the  reader  to  the  conviction  that 
'no  psychological  meaning  can  be  attached  to  the  word  genius. '  .  .  .  While  all  men  of 
genius  liave  common  traits,  they  are  not  traits  characteristic  of  genius:  they  .ire  such 
as  are  possessed  by  other  men,  and  more  or  less  by  all  men.  .  .  .  Dr.  Hirsch  believes 
that  most  of  the  great  men,  both  of  art  and  of  science,  were  misunderstood  by  their 
contemporaries,  and  were  only  appreciated  after  they  were  dead." — .^liss  J.  L.  Gilder, 
in  the  Sunday  li  'or Id. 

"  '  Genius  and  Degeneration  '  ought  to  be  read  by  every  man  and  woman  who  pro- 
fesses to  keep  in  touch  with  modern  thought.  It  is  deeply  interesting  and  so  fiiU  ot 
information  that  by  intellectual  readers  it  will  be  seized  upon  with  avidity."— BuJ^aU 
Commercial. 

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p  OP  ULAR  ASTRO  NO  M  Y.  A  General  Description 
-^  of  the  Heavens.  By  Camille  Flammarion.  Translated 
from  the  French  by  J.  Ellard  Gore.  With  3  Plates  and 
283  Illustrations.     8vo.     Cloth,  S4.50. 

"The  fullest  and  most  elaborate  compendium  of  popular  knowledge  of  astron- 
omy. .  .  .  The  book  might  reasonably  be  pronounced  the  most  desirable  of  its  kind." 
—New  York  Sun. 

"M  Flammarion  has  produced  a  work  that  charms  while  it  interests.  He  has 
classified  ^istronomy  so  perfectly  that  any  person  of  ordinary  intelligence  may  learn 
from  his  book  prartically  all  the  men  in  the  observatories  know."' — New  York  Times. 

"Flammarion  talks,  and  his  conversation  is  free  from  those  teclinical  expressions 
which  make  the  obscure  style  more  obscure.  He  treats  the  most  abstruse  problems  in 
such  a  faslii  ,n  that  you  see  through  tliem  more  clearly  than  you  ever  thought  it  pos- 
sible to  do  without  years  of  study."— iV?w  York  He' aid. 

"  While  the  tianslator  has  done  excellent  work,  he  hds  also  added  largely  to  the 
value  of  the  book  by  his  carefully  prepared  notes,  in  which  he  brings  every  astro- 
nomical theme  down  10  date." — Chicago  Iiiier-Ocean. 

"The  l)ook  is  one  of  extreme  interest,  and  to  our  mind  far  surpasses  in  fascination 
any  novel  that  was  ever  written." — London  Literary  World. 

/1STR0N0MY  WITH  AN  OPERA-GLASS.     A 

-^^^  Popular  Introduction  to  the  Study  of  the  Starry  Heavens  with 
the  Simplest  of  Optical  Instruments.  By  Garrett  P.  Ser- 
viss.     Svo.     Cloth,  $1.50. 

"  The  glimpses  he  allows  to  be  seen  of  far-stretching  vistas  opening  out  on  every 
side  of  his  modest  course  of  observation  help  to  fix  the  attention  of  the  neghgent,  and 
lighten  the  toil  of  the  painstaking  student.  .  .  .  Mr.  Serviss  writes  with  freshness  and 
vivacity." — London  Saturday  Review. 

"  By  its  aid  thousands  of  people  who  have  resigned  themselves  to  the  ignorance  in 
which  they  were  left  at  school,  by  our  wretched  system  of  teaching  by  the  book  only, 
will  thank  Mr.  Serviss  for  the  suggestions  he  has  so  well  carried  out."— New  York 
Times. 

"  We  are  glad  to  welcome  this  popular  introduction  to  the  study  of  the  heavens. 
There  couUl  hardly  be  a  more  pleasant  road  to  astronomical  knowledge  than  it 
affords.  ...  A  child  may  understand  the  text,  which  reads  more  like  a  collection  of 
anecdotes  than  anything  else,  but  this  does  not  mar  its  scientific  vA:\e."—Natnre. 

"Mr.  Garrett  P.  Serviss's  book,  'Astronomy  with  an  Opera  Glass,'  offers  us  an 
admirable  handbook  and  guide  m  the  cultivation  of  this  noble  esthetic  discipline  (the 
study  of  the  stars)." — New  York  Hojne  Journal. 

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TROUGH  MAGIC  GLASSES,  and  other  Lectures. 
A  Sequel  to  "  The  Fairy- Land  of  Science."  Illustrated. 
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CONTENTS. 
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"  It  is  certainly  a  useful  and  convenient  volume,  and  readable  too,  if  we  judge  cor- 
rectly of  the  degree  of  accuracy  of  the  whole  by  criiical  e.xamination  of  those  cases 
in  which  our  own  knowledge  enables  us  to  form  an  opinion.  ...  In  general,  it  seems 
to  us  that  the  handy  volume  is  specially  to  be  commended  for  setting  in  just  historical 
perspective  many  of  the  earlier  scientists  who  are  neither  very  generally  nor  very  well 
known." — Afeiu  "i'ork  Evening  Post. 

"  A  wonderfully  interesting  volume.  Many  a  young  man  will  find  it  fascinating. 
The  compilation  of  the  book  is  a  work  well  done,  well  worth  the  doing." — Philadelphia 
Press. 

"  One  of  the  most  valuable  books  which  we  have  received." — Boston  Advertiser. 

"  A  book  of  no  little  educational  value.  ...  An  extremely  valuable  work  of  refer- 
ence.' ' — Boston  Beacon. 

"  A  valuable  handbook  for  those  whose  work  runs  on  these  same  lines,  and  is  likely 
to  prove  of  lasting  interest  to  those  for  whom  '  les  documents  humai  • '  are  second  only 
to  history  in  importance — nay,  are  a  vital  part  of  history." — Boston  Transcript. 

"  A  biographical  history  of  science  in  America,  noteworthy  for  its  completeness  and 
scope.  .  .  .  All  of  the  sketches  are  excellently  prepared  and  unusually  interesting." — 
Chicago  Record. 

"One  of  the  most  valuable  contributions  to  American  literature  recently  made.  .  .  , 
The  pleasing  st\  le  in  which  these  sketches  are  written,  the  plans  taken  10  secure  ac- 
curacy, and  the  information  conveyed,  combine  to  give  them  great  value  and  interest. 
No  better  or  more  inspiring  reading  c  mid  be  placed  in  the  hands  of  an  intelligent  and 
aspiring  young  man." — New  York  Christian  ll^ork. 

"  A  book  whose  interest  and  value  are  not  for  to-day  or  to-morrow,  but  for  indefinite 
time." — Rochester  Herald. 

"  It  is  difficult  to  imagine  a  reader  of  ordinary  intelligence  who  would  not  be  enter- 
tained by  the  book.  .  .  .  Conciseness,  exactness,  urbanity  of  tone,  and  interestingness 
are  the  four  qualities  which  chiefly  impress  the  reader  of  these  sketches." — Buffalo 
Express. 

"  Full  of  interesting  and  valuable  matter."— 7"A*  Churchman. 


New  York :  D.  APPLETON  &  CO.,  72  Fifth  Avenue. 


Date  Due 


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